It is difficult to carry a pot, or even a metal cup, in a pocket size survival kit. If I have any sort of pack or bag I do carry one. There is also the issue of chemicals and heavy metals. In the area where I live, unless you get up into the mountain, you are risking farm and industrial waste in any ground water. Something small that can get you through far enough to help or improved water purification capabilities would be a good thing. Something small is more likely to be on you and for you to have it when you end up in a survival situation. Not all survival situation happen in remote and pristine wilderness.