Cisco Nexus Fabric Extenders with 10G-SFP in path

Keep in mind that a FEX inherently is a method by which to extend the fabric, thus the name. The ability to manage centrally while still having "line cards" distributed throughout the DC is the real reason to use a FEX. Drastically reducing cabling is valuable to anyone, technical or not, and the argument of being able to manage the entire infrastructure at fewer points is time savings, pure and simple.

One of your big doubts is that you're worried about single points of failure. All devices have the ability to be dual-homed, and in certain configurations, you can even establish virtual port channels with the Nexus 2K FEXs themselves.

Take a look at Cisco's documentation. You'll find that you can design a topology that is just as redundant as the "direct-cable" option you're considering, with less fuss.

The Nexus 2k isn't an actual switch, but a "fabric extender" - it functions as a separate, self-contained linecard for the Nexus 5k. The following CiscoLive PDF has great details on the Nexus 2k/5k architecture

It's clearly pointed out in page 56 that the 2k does not perform any local switching.

The Nexus 5k is what performs all of the forwarding, integrating the Nexus 2k fabric extenders remotely into its backplane via IEEE 802.1BR.

As a future point of advice, googling for "a day in the life of a packet on [x]" where [x] is your favorite piece of gear will often produce good documentation on the architecture of that gear. YMMV with this however, as not all vendors make this information public.