I think that no one wants to believe that DD was so horribly misguided trusting Snape, so Snape has to be a good guy.
In the past week or so I've re-read the first 5 and finished 6 this morning. Snape HAS been horrible to Harry, but in every instance I recall, something good has come from it.
In 6, Harry having the Saturday detentions allowed the Quiddich team to play without him -- and win. It seems to me that JKR is trying to make the rest of characters more real, and I predict that in 7, R, H, G and possibly even Neville will have confrontations with V apart from Harry.
That reminds me- I was so disappointed that there wasn't more Neville! He's one of my favorite characters. I was so happy to read that he got one of the last wands Mr. Olivander sold.