Based on answers to this SFF question, it seems that Quirrell was not a very good candidate for DADA professorship - a book-smart "delicate" guy.
Yes, he was - as per Hagrid - "brilliant". But if there is one class where practical proficiency was very important compared to academic smarts, DADA is it.
Why didn't Dumbledore appoint someone with more experience actually fighting the Dark in the First Year? Alastor Moody (or some other retired Auror), or one of the existing teachers except Snape? (Professor McGonagle, or even himself for that matter)?