Personally I think Snape is really not that good in DADA. Also it is a rather important subject with Voldemort still out there, and Snape seems to only sponsor Slytherin students. On the other hand, Snape would have been a whole lot better than Umbridge.
I have no idea, actually, just thinking out loud.
Greetings, Lilah ------ It's only bad, when you don't notice anymore that you are crazy.