Plus all those plastic bottles (most of which never get recycled) are horrible for the environment. I just use a Brita water pitcher and reusable containers.
Tap water has all kinds of drugs in it from people flushing their pills or simply taking a frothy piss, so you get free water AND free drugs. Even if you pay for your tap water every month like we do, you're still getting free drugs and that's nothing to sneeze at.