Ummm.. tons and tons of threads about this. I think the most recent one is "Snape and the DADA" but I'm not positive... anyway...
I think Dumbledore keeps Snape from being the DADA teacher in the same kind of way as you don't take a recovered alcoholic to a bar. Even if you trust that they won't drink, you don't give them the option. I think Dumbledore wants to keep Snape away from anything where anything related to Dark Arts and the kids at the school are together... Not that he doesn't trust him, but just as a precaution.
I do think, however, that in book 7, Snape will do something to prove himself, and at the end of book 7 will get the DADA post for the following school year (opening the job of Potions professor for someone... I think Hermione, but I'm alone in that thought, I think
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