Marketing.
7.2K drives are slower and easier to produce, and with higher error thresholds which improves yields (and capacity). However, in terms of I/O operations each discrete disk can support, the 7.2K drives are markedly less performant than their faster brethren. Therefore they get the 'Nearline' moniker, as they'll hit I/O saturation much faster than an equivalent number of 10K or 15K disks. Therefore the storage producers need a way to convey, "we have faster stuff," so they went with MDL/Nearline.
This is how they try to encourage people who need both fast and lots of storage to go for the faster drives. Those on a budget will see that you can get (for example) a 1.5TB MDL/Nearline for half the price of a 450GB 15K drive and wonder why the upcharge. Even so, 48 7.2K RPM drives will still outperform 12 15K RPM drives. It's just that the 48 7.2K RPM drives will probably have a capacity of 30TB, where the 12 15K RPMs may only have 5TB of capacity.
Which is another way of saying...
Go 7.2K RPM when capacity is your number one goal and performance not really a goal.
Go with 15K RPM when performance is your number one goal, and capacity secondary.
Best Answer
Hot-swap or non-hot-swap, plus a slim one for blades.