From time to time I scan the bounty tab, and being the pessimist and grouch that I am, I generally find two or three awful questions that, in my opinion, deserve to be closed. But I can't even register a close vote because the bounty protects them.
A bad question is no better because its author has offered a bounty.
I've failed in the past to persuade that 5 close votes should close a question, even with a bounty, refunding the rep to the asker.
So, I've another idea. Don't allow bounties on negative-total-vote questions. This would at least stop the most egregious examples. If I were duke, I'd go further, and require a question to have attracted a net positive score before it can have a bounty, but I doubt I'd ever win that argument.