There are plenty of questions that are completely answered in the comments. If this happens to one of your own questions, you can follow Qiaochu's advice here: What should one do when one's question has been answered in the comments?.

But if the user obviously left M.SE (for whatever strange reason) without following this advice, the question will be bumped by Community♦ from time to time without use. These questions also mix among the posts, when you search for unanswered question.

So my question is: Would an additional reason to close/flag a question, something like "user abandoned, answered in comments" help to keep the system clean(er)?

Might be that closing is not the right thing, since it will prohibit future answers to be given. I just thought that this extra-ordinary "accept" step should be done by several members, as it is done for closing.

$\phantom{PS: Answers to this question in the comments will not be accepted!}$

• The question is not bumped if it has at least one answer with one upvote (or accepted), see here and here. (This is already explained in Willie Wong's answer, I just wanted to add some pointers to places where this information - and perhaps some further details - can be found.) – Martin Sleziak Jul 24 '12 at 8:37
• and to quote your 2.: [Community] uses the 0-upvoted answers definition. The community user will not bump a question if there are any answers with a score > 0 – draks ... Aug 2 '12 at 18:28