The space-saving thing is a bit misleading. Yes, they take up no space when they're empty. But, when they're full, the need the same amount of space as a hard bottle. Now, it's normally only for a few hours, so in a well-planned load, you'll be able to accomodate it: a sleeping pad can strap on the outside for a few hours, you'll have an expansion sleeve that can be expanded, or you can lash the water to the outside of the pack.

But, if you're going to leave the extra space in your pack in a "permanent" way, say by using a 60 liter pack instead of a 50, then the space saved (or "reserved" for the full bottle makes collapsibility somewhat extraneous.

Having said that, I will also say that I carry collapsible bladders regularly. I don't use bottles, per se; I use an MSR Dromlite, which (filled with air or water) makes a really nice pillow.