Seems like a mixed blessing to me. If you take sandals every time you go, without exception, it might be a decent idea. However, if you only take sandals sometimes, would your design lock you into taking them every time - in effect making the pack heavier than it would be otherwise?

Also, it seems that the times you would most need the sandals to protect the bottom of the pack would be when it's on the ground. It's on the ground the most - and most likely to get pushed or pulled - in camp. And I'm assuming that in camp is when you'd be wearing the sandals, in which case they provide zero protection for the pack.

It's really an innovative idea, just be sure to think about how the pack gets used before committing to it. (By the way, I seem to remember that a lot of the ultralight packs, like Gossamer Gear, don't bother with a heavier weight fabric for the bottom; instead, they caution the user to treat it somewhat gentler than an airline baggage handler would. It's been a while since I've seen some of these packs, so don't take my word for it.)