May be it's the wording, but, we can close a question as being unclear or too broad even if the question has been answered (and marked as answer)
If a question has been answered and marked as answered, I'm not totally convinced it can be considered "unclear" or "too broad"! If anything, it had to have been clear to at least 1 person due to the fact it's been answered and it can't be too broad because it has been answered. :)
Updated, to add "too broad" to the equation.
I've had the same thought on a number of questions. In many cases, I've been convinced that the answer was a lucky guess that turned out to be right.
Super User isn't about answering for the OP, it's about building a knowledgebase; the OP gets their question answered in the process. It's great for the OP when they can get what they are looking for despite the question being unclear. If it is unclear, though, it will be of limited use to others.
If the accepted answer provides enough information to clarify the intent of the question, the question can be edited. Sometimes, though, even the accepted answer isn't enough to turn the question into a useful one (still ambiguous issues, etc.). In that case, it isn't helpful for other people searching for a solution to their own problem. It also invites a collection of confused answers, and users wasting time writing answers based on potentially wrong and varied interpretations. I don't see an issue with closure.
Edit: Just to add a clarification in relation to Psycogeek's answer -- our answers address different issues and are not mutually exclusive. The question isn't about deleting unclear questions, it's about closing them. If an unclear question has an accepted answer, as described in the question, closure won't result in deletion. If I understand correctly, it is the same case if the answer is upvoted but not accepted. Closure just prevents new answers until the question can be made clear so that it is answerable by other than a lucky guess. I agree with Psycogeek that a bad question with a useful answer should not be deleted, but I think it can be appropriate to close it until it is made clear.
"There are no bad questions if they have great answers"
When your searching for clues and information via a web search, you may search as badly as a questions was asked, you may search just like a bad title reads.
Having a great answer under any question, that would be found using a search engine, however badly one searches, can still end in reading great information, based on very well written answers.
Should bad questions be closed, probably.
Should great answers be deleted , probably not, especially when people take their time and effort to create good content, to only have it destroyed by others.
In the debate sense to Fixers answer bad searches, looking for good info, will bring up SU bad questions, if they still have good answers (not just accepted) the knowledge base is still available. If they are tossed, or rightly if anything is tossed , where then is the knowledge?
I take the other side of this debate, but still maintain/agree that indeed there is both questions and answers that do not deserve life. So as always make good judgment calls, as there is no ONE way, there is people making decisions.
