I have yet to see any practical use for the rear touchpad, and I doubt I'll ever see one. The duel cameras are only useful if you're planning on using video chat or using the Vita as a video/still camera (why?). The game included in Welcome Park where you can take pictures of objects that look like faces and bring them to life is kinda fun to screw with, but I doubt we'll ever see a truly good use for either camera in a serious game.
I feel like the Vita is trying to be an all-in-one portable electronic device for communication, video, music, photos, etc. Of course, they have to put all that superfluous crap in there too, just because they can. Problem is, that's what EVERY other device is trying to be these days. And I'll bet that most who own Vitas already own 2 or 3 other devices that can do all of those things just as well. I can maybe see the use for the front touchscreen and motion sensor. Not the other features so much. The people who are going to buy Vitas are gamers who want something close to the PS3/360 experience in a portable device. That's what they should be focusing on - not trying to market it as another alternative to a smartphone, tablet, ipod, etc.