In the last 10 hours, I've had two accepted answers that have been deleted by the (same) post author. Since my answers were the only ones and they weren't upvoted (maybe because the deletion took places minutes after my answers), the post author could delete its two questions.
Now I am not whining about the loss of reputation, but what annoys me is that I took the time to write detailled answers, which were of sufficient quality to be accepted, but they were deleted nonetheless.
What I would suggest is that in order to delete a post, the condition "has only one answer, but that answer has no upvotes" becomes "has only one answer, but that answer is not accepted and has no upvotes". I know that the post author could unaccept an answer before deleting his post, but that could be a way to remind them that stackexchange is not only a forum to ask and be answered, but also a database of content that can be used by future users.
Another suggestion: how about blocking deletion during the 24h after an answer was posted? That way, it has a chance to be upvoted.