I prefer a single wall metal (ti or alum. or SS or even tin) cup to a plastic cup. Say something happens to your normal pot that you boil water in while out in the wilderness (maybe you lose it or it gets crushed beyond repair in the field, etc.). That plastic cup is pretty much worthless now over a campfire/stove/whatever, right? I always like to have a "back-up" when I'm far from home...especially when it weighs so little.
But then again, YMMV. <img src="/forums/images/graemlins/smirk.gif" alt="" />