The total amount of funding for schools hasn't changed. One could inversely argue public-school funding is bad because it reduces money for private schools. Both arguments are tautological. They first assume their system is better, say the other competes for their money, and thusly conclude that funding for their better system should be preserved. (This is not an argument for or against charter schools. I'm just saying the "depletion" argument is silly.)
> You also divert money from public schools for no guarantee of better results
When properly done, charter schools can outperform public schools [1]. Otherwise, the evidence is mixed. I'd thus argue defaulting to the status quo. Teachers argue the status quo is state education while conservatives argue it's parent choice.
[1] http://www.huffingtonpost.com/2013/06/25/charter-school-perf...