If I was desperate for work, then I guess I might think about it. However, I'm not (desperate for work that is)! Being quite mercenary about it, I would end up being a bit resentful of knowing that I could "let" all my places for full fee, but that I was charging less. I guess that is being a little unfair (but I have got twins of my own, so I'm allowed to be unreasonable!)
I pay out quite a lot during the week for activities that we attend, and don't expect parents to cover those.