That's true, a donation should not even implicitly be for anything in return - a chipin style system is more of a collaborative payment for a service rather than a simple donation in the true sense of the word (ie, not expecting anything whatsoever in return). If people expect a certain feature to be finished or worked on after contributing to a fund, that's not a donation, it's a payment, because naturally the contributor would expect to see some kind of result.
I'm not totally shooting down the idea; I think it's worth bearing in mind, but maybe to overcome this donations/payment split, it could be hosted on a different site as a more commercially oriented project. Then there would be no confusion whatsoever as to which is which.