I have never had a customer give me trouble over using their water...................and my policy is I leave the job with the same amount (or less) water then I came with. If I roll in with half a tank, I try to leave with half, and not take extra from the customer. And if you do, so what, its only .05 cents a gallon.
I fill my tank sometimes at the local water company.........they have a two inch hose, and it fills my tank in about a minute and a half!
I am washing a house in the next couple of days that has a bad well, and I will not have any onsite water.................guess that answers your question about what a tank is good for. Also, what the others said about having a bypass set up is nice too.