If the big bang didn't happen, then how could evolution happen?
If the Big Bang had not happened, nothing could have happened. But that doesn't mean that the Big Bang is a theory in computer science or home economics, despite the fact that the existence of a universe is a necessary condition for those disciplines.
Evolution is a theory in biology; the Big Bang is a theory in astronomy. The division of the disciplines is a useful tool for breaking down big problems, and it's not something to just throw away.
I can accept that the term "big bang" may not be accurate and is simplified, but if science just uses labels to help describe things (and those labels are not the whole theory) then why not accept the label of "God" to help use understand things.
Because the use of the word "God" is not helpful in creating understanding. In fact, it creates misunderstanding - the Protestants will think you're talking about how they understand God, the Catholics will think you're talking about how they understand God, the Muslims will think you're talking about a radically different conception of God altogether. There are hundreds of thousands of individual religions, all because people can't compromise or agree on their conception of "God."
So, as a word, it's worse than useless. It's the opposite of communication to use the word "God."