We have used Platypus brand "flat" water bottles while backpacking, and my previous manager borrowed mine for a one month backpacking trip for her family through China. They survived just fine. They were abused, tortured, beaten up, boiled, stomped on, dropped, tossed. You name it, it happened. They still are water tight and look just a little bit barely worn.
I do NOT recommend doing same to cheap knock-offs.
-SW