Snape Revisited

Just something that coalesced in my brain pan while listening to HBP, on whether or not Snape is a poison or a wine in the DADA potions riddle from Book 1. The litmus test (or at least one of them) has been, I think, agreed upon, as: "What did they teach as far as DADA? Was it good or bad, helpful or hurtful?" In looking at Snape's performance as DADA professor on this particular level I still have to come down on the "poison" side, because of 2 things. 1. We know from HBP that Harry expects to get bad marks on his essay on defending against Dementors because he disagrees with Snape on the best approach. We also know Harry is very good at defending against dementors because he drove off a hundred at once, and that casts Snape's insistence on his own approach in the "bad" category - it detracts from truly learning how to defend against dementors (and if Red Hen is right, this may be THE central question of the series in regards to DADA). 2. In HBP we also hear Snape say "and they [the essays] had better be better than that tripe I had to endure on resisting the Imperius!" Again, We know (from the central book in the series, according to the Chiastic structure theory, book 4) that Harry can resist the Imperius curse of Voldy himself, the most malicious and bent-on-control wizard of all time. Most of that class learned what they do know of resisting the Imperius from Harry's experience with Barty Jr/Moody in class and from Harry in the DA. Once again, Snape seems to be detracting from the students learning valuable lessons on DADA. Of course, that particular litmus test would cause us to rethink Barty Jr, I guess. We refer to him as a poison because he took Harry to Voldy, not because of his class lecture content. That's why I say I think it is only one of the litmus tests. Just some thoughts....