My original answer (below was based on my incorrect assumption that Arik was suggesting using the wiring inductance in a controller to replace formally provided inductance. In fact, he is saying that in the controllers of interest there is NO formal inductance, and he was wondering if the wiring inductance served a useful role overall.
Simple PWM can be used to vary the current which a PV panel will deliver and to control bttery voltage. It can act as current limiter, constant current controller or voltage controller.
A PV panel used without an energy converting controller, suh a an MPPT controller, usually acts much like a CC (constant current) source. This is because Vmp (Voltage at max power) is > Vbattery under most sun conditions and the panel is loaded with a lower effective resistance load than is optimum. An MPPT controller increases the effective load resistance so the supply voltage can rise to the optimum value.
If you connect a PV panel to a battery then current flow will redcue as PWM duty cycle redcues. It will not be a linear reduction as vpanel will rise as load is reduced, tending to act against the current reduction from the PWM. However, in practice you can set current to any value equal to or lower than what ypu'd get with a hard connection.
If you want to limit battery voltage to a certain value then simply reducing or stopping current flow when the voltage is high enough, will work as a "constant voltage' source.
current
Simple MPPT can be little more than the buck converter I outlined plus a controller.
By doing no more than holding panel voltage at about 80% - 85% of Voc_panel_full sun you will get very close to true MPPT performance.
Second addition
As the question evolves, so can the answer :-).
There is no doubt that simple bang/bang on/off control is undesirable and causes undesirable battery current and voltage variations. My comments about the controller being able to control voltage are true over a long time period relative to a PWM cycle but all sorts of interesting stuff may happen over a single cycle or a small number of cycles.
Adding an inductor allows energy storage and smoothing - an existing controller MAY be able to be "improved" by just adding an inductor and flyback diode and maybe one or 2 reservoir caps depending what is there now BUT the existing control circuitry may have a fit (or not)
due to the changed response. It would probably in many cases to use the existing power level hardware with L,C,D as requisite plus either new software or (possibly more easily) a new control core. An MPPT controller needs cost little more than what is there now. Pricing is often controlled by "because we can" and "because they can't" factors.
Having the series switch (probably MOSFET) in linear or resistive more would help make behaviour nicer at the expense of power dissipation in the switch. The heatsink size is uncertain as can only be seen end on but it looks substantial. If the switch is run as a resistor then it COULD be setup to operate a setady current feed to battery. If desired this could be only done in holding mode where current is low. eg in Panle V_light)load is say 17V and Vbat hold is say 12.6V and Itrickle is say 100 mA then dissipation in a FET in this mode is P = V x I = (17 - 12.6) * 0.1 = 0.44 Watts = minimal. If you could sink say 5 Watts and needed to provide current from 18V to 12V you could have I = W/V = 5/(18-12) =~~ 800 mA.
Using on/off PWM is non ideal and will lead to waveforms something similar to those shown by Arik in his second edit. The magnitude of the spikes will depend on how much capacitance is present BEFORE the switch, to a lesser extent how much capacitance after the switch, wiring resistance (and to some extent inductance) and battery characteristics and state of charge and, importantly, PWM frequency. Arik has shown the signals as step changes at switching boundaries followed by linear ramps. I would expect the step changes to be modified by effect of capacitances and linear ramps to tail off into more or less steady state flat spots as PWM on or off time became long relative to battery & PV panel settling down to steady state under the given conditions.
I do not show a capacitor on the PV panel in my outline schematic below but if there is one then the PV panel will slew more slowly and can be held near a constant voltage more closely. This would limit the more objectionable spikes and excursions shown by Arik.
Also, an ideal battery may exhibit step change and "instantaneous steady state" conditions as suggested but it is likely that in real life you get more complex responses- an oscilloscope would be your friend here.
Original answer - useful but not what was asked for.
It is extremely likely that you circuit diagram is incorrect and that the simple PWM controllers are Buck converters as shown below.
Wiring inductance could notionally be used but in practice is too small to use in sensibly practical converters. Resonant frequency is not the critical factor. At resonance Vcap would swing 'widely'. What is required is an inductor such that delta V is small during the on cycle - perhaps 1V p-p and ideally quite a lot less. Using wiring inductance would probably require MHz range switching and would be likely to produce ill defined low efficiency high RFI situations.
With a suitable controller such a circuit can provide constant current or current limiting or voltage controlled output.
D1 is usually either a Schottky diode or a synchronously controlled FET switch.
Plenty of things have multiple batteries.
Some hybrid cars have their traction battery pack and a "standard" 12v battery for the normal vehicle systems, including starting/charging the internal combustion engine.
High reliability UPS systems (telecomms/data centre/any critical service) may have multiple battery arrays, typical installs I've seen they have 3 of everything (so 3 identical sets of UPS + battery array + control + monitoring) of which any two can handle the full load if one of the three dies - and usually a backup generator, for large loads they do the same again and have 3 generators, any two of which meet the required capacity.
That said I've seen those things chained in far larger numbers; A suite of maybe 10+ racks of triple-redundant UPS+battery etc., so each rack is its own self-contained 2-of-3 redundant UPS and the 10+ racks share the load with a redundancy of maybe 20%. Sounds like overkill but plugging in multiple duplicate UPS racks is an easier build and scales easily. Likewise, 3 big-ass generators side-by-side are easier to install than one supertanker-size one.
Also, the telco I worked for there were backups to the backups - the UPS's to catch blips in the power / hold things up while the generator comes online, the generator for short to medium outages (usually fuelled for a week or so continuous running, location dependent), and remote monitoring that dispatched a power guy ASAP who could also bring in a mobile generator, additional fuel bowsers, etc, etc... gotta love a system built during the cold war to be nuke-proof!
The point in critical/high-reliability/disaster-proof systems is redundancy, no single-point-of-failure, preferably modularity too. Aside from the backup generators, which tend to live in one special room, generally the UPS's are separated - either per rack / suite of equipment, per floor, per supply or whatever so if one bit of kit goes bang it doesn't bring the whole place down with it.
Emergency vehicles (or overlanders, campers, etc.) have twin or triple battery systems, usually one to take care of the starting & running and one or more to power the "living" systems, with the "emergency" option to use the "living" battery to jump-start the "main" one.
Some vehicles have 12v and 24v systems and rather than do the bodge of pulling 12v from one half of a 24v pack (recipe for problems) they add a secondary 12v battery (and sometimes an extra 12v alternator).
There are also more subtle examples: Many devices that keep time have a real time clock (RTC) chip which will have either a small supercap or watch-battery to power it independently of the main system power, be it mains/wired or battery (EG laptop, digital camera) you'll find you can unplug the device & remove its battery and when you reconnect it it will still remember the correct time. The RTC battery has very different properties to the main system battery - often they are not designed to be replaced, only supply a tiny current (microamps) at a low voltage to keep the RTC oscillator ticking over.
Edited to add: I found some photos of the aforementioned multiple-battery setups:
Both setups are floating at -48v, the copper & blue bars are busbars.
Best Answer
Sometimes you see electrolytics in series to increase the overall voltage rating: -
Here you see five 50V rated caps in series. Note that bleed resistors are needed to equalize the voltage sharing evenly.
Having two series strings of capacitors is just a means of doubling the value.