From by my own experience, you'll have to review/test the boring task from a one-time contributor, which is also not fun. You don't need to do that as often if

From by my own experience, you'll have to review/test the boring task from a one-time contributor, which is also not fun. You don't need to do that as often if the contributor is going to stay, but you're not attracting this sort of contributions with bounties. The last point is also similar. If I get a significant contribution in area where I'm not competent, I might take it, but is it going to maintain or fix problems to it in the long term? I'd rather not include a feature which I know I can't maintain long term.