There are rather strict measures in place to prevent self-deletion of answered questions: in particular, as soon as an answer has just a single up-vote (note upvote, it does not even have to have positive score) the questioner cannot delete anymore.
I am not sure we should make them still more strict and basically prevent any and all self-deletions of answered questions.
Moreover, in general it is certainly discouraged to self-delete answered questions of reasonable quality. If you run into such a situation, signal it to a moderator, via a flag on the post, by posting in the reopen and undeletion request thread on meta, or in the moderators' chat room if you have trouble accessing the post.
Usually, it will be undelete if the deletion seems ill-motivated.
All that said, given the wording you use to refer to the questions, it is not clear what is lost by them being deleted. If the question was poor, possibly stated incorrectly, and the answer completely routine, why keep them around? Maybe they should be deleted, maybe they would be deleted anyway later by community moderation anyway. Note that deleted content counts towards automatic posting bans. Thus, a user can not do this that often or all the time.
To sum it up: if there is a Q&A pair of alright quality that got quickly self-deleted by questioner, signal it and it will be undone. The technical measures in place are already pretty rigid, I do not think it is a good idea to make them more rigid. If the quality is however rather poor, I do not see the problem with the deletion.