Well, since the late '60s I've always understood the concept to be saving weight and bulk by using clothing, jackets, packframes or whatever to keep your feet up off the ground. In warmer weather you might be able to do without, but it's still more comfortable to have some padding. You usually end up using something like that for a pillow on the other end, anyway. Full-length pads mostly get used in the winter, when, if things get bad, you might want to wear everything available.

Most types of pads come in both lengths, but some foam pads are cheap enough that you just buy the one length and cut it to whatever you need/want. The inflatable pads can't offer that option, so they have to offer multiple lengths.

I find it worthwhile to carry one cotton stuffsack to use as a pillow- much more comfortable against the face than nylon, and it slides around less. A friend uses a large fleece bag, and if it gets really cold he shoves his feet inside it, inside the bag- but it's bulky.