The reason that Dumbledore never allowed Snape to be Defence Against the Dark Arts teacher was because the position was cursed so that no teacher could last more than a year, and Dumbledore wanted to keep Snape around, wasn't it..?
As I said, I may sound a bit dense by only just realising this, but still... I love little intricate details like that. The whole plot has sense and reason behind it.
Then again I could just be making this up










