I understand that the bounty system is like "renting ad space"; however, I'm wondering if maybe this is worth rethinking.
For certain questions, I know some users have posted an initial question on Stack Overflow, and they have taken the time to thoroughly research/ask question on other forums such as gxt, gwt, jboss and Google. Unfortunately, sometimes the questions are way too narrow. I don't think it would be an issue to have an extremely specific problem posted here with a solution.
I believe it may be worth allowing bounties to stay open until the OP deems the question answered. As time goes by, an user accrues more reputation, and they can increase the bounty on their question. The question will then attract more and more attention over time until hopefully somebody answers it.
I have spent all of my current reputation trying to get a question regarding Google PlayN, and I have taken the time to find the answer of Google+ communities and the Google PlayN forum. I have had no luck, and now I can't rebounty my question because I do not have the required reputation; therefore, my question will most likely not be answered unless I can get the necessary reputation.
I kind of picture it like an old time western where the bounties stay up. It doesn't seem like over time the person just gets the charges dropped. It would make more sense to me that if the person is willing to push bounty to a question he/she should just be able to keep adding more to it. Often times, I only make a post if I've exhausted all other resources - and normally the post is pretty obscure (which are the posts I normally find the most useful as you can read from other people exactly the methods they tried, what does/doesn't work for them, and the pitfalls of doing things a certain way). Posting a question like this may take some time, but if the bounty is greater, you may get more people trying to answer the question in hopes of getting the bounty. This will increase the chance of you getting a solution faster.
In summary, I think it would make more sense if the question stayed open instead of just expiring after 7 days. I don't really understand why it is "like renting ad space". I think allowing people to just keep pushing points will make the bounty system more like a real economic system rather than a gambling system.