I have a couple of packs that use one's sleeping pad in a pad pocket to provide a frame. This works OK, for me, as long as I don't go with much more than, say, 15 lb total weight. But, a folded pad in a pad pocket next to your back used as a frame has several drawbacks. First, it moves the center of gravity of your pack away from your back by the folded pad thickness. With a light pack this is inconsequential but right after a resupply stop or with a heavy load of food and/or water it makes for an uncomfortable pack and a lot of pull on ones shoulders. Second, a sleeping pad is moisture impermeable so one sweats up a storm between pack and back. Finally, if you use a self-inflating pad, there is more wear and tear on the pad. I have had two leaks show up along fold lines on my pad after a week of use as a frame; YMMV.