So I just added a bounty to my own question because I need an answer pretty soon if it's going to be useful to me. Not the most altruistic use of the bounty system, but I figure it's my rep, I can be selfish with it if I want :-) My only qualm is that I had to pick a reason, and none of them seem to really fit “I just want an answer badly enough to sink some rep into it.” I went for “This question has not received enough attention,” but that seems like a whiny thing to say about your own question ... maybe it's received only as much attention as it deserves! IOW, every reason on the list has to do with the merits of the question when (as I see it) a legitimate use of the bounty system is, in the grand tradition of capitalism, to realign incentives in your favor (for a price).
Maybe the best solution would be just to allow not giving a particular reason. I mean, putting rep on the table kinda speaks for itself; if the purpose is really just the obvious “here's some rep if you can answer the question,” why obscure it?