I have no problem with it. Like any business (non-profit or not), it does cost money to run Wikipedia.
One can choose to donate or not. The alternative is that they may have to eventually go to an ad-based revenue model, and, I don't know about the rest of you, but I sure get tired of seeing ads everywhere. I use Wikipedia quite a bit, and if I can kick in a little money once in a while to help them avoid that scenario, I really don't mind.