My observation (made in German language WordPress forums) is that there are many not-quite-so-competent "webmasters" around looking for help in topics that they do not really master.

Right after having gotten competent help and/or a working solution for their problem, they want to have the whole thread (including the solution) deleted in order to hide the fact that they were looking for help in the first place. They obviously do not want to see their "limited competence" displayed in public.

I'm afraid that allowing forum users to edit (or even delete) their postings over a longer period of time would probably result in more obscurity than in more accessible knowledge...