Bounties are just to give more visibility to a question, and get an answer when you don't get any answer that explain what you want to know. As you are giving to the users an incentive to answer, it is more probable you get an answer, or a comment; editing the question to improve its quality doesn't happen more often on questions with an open bounty.
There are two categories of users: who sees the question in the "featured" tab, and who sees the question in the front page, all times it is bumped by a new answer, or an edit for an old answer. The first group looks for questions with an open bounty to answer them, and they are more interested in giving an answer; the other group could edit the question, if there is something to edit, but that is not done because the question has an open bounty.
There could be somebody who gives a quick answer just for the matter of trying to answer a question with a bounty. Considering some answers I have seen, I would say the overall quality of the question (answers and question) could get lower. Those users are surely a minority, but for some questions the number of guesses increases when the OP offers a bounty.