Many people seem to assume Snape only pretends to actually want it for some spy-related reason. Personally I'm undecided on the matter, but somehow I find it hard to imagine the man who waxed poetical about the beauty of the simmering cauldron actually wanting to teach foolish wand-waving instead. Perhaps JKR will soon give us a reason.

And I'm rather tired of the fact that every time someone tries to introduce a new adult character he/she always winds up being the new DADA prof, so I wouldn't mind that pattern broken by giving the job to a familiar character instead!