So I read this theory the other day about how Lupin was replaced by a Polyjuiced Wormtail between OOTP and HBP. This theory made a few relevant points but overall, I think the real question is, what would this accomplish? Not really anything relevant, except that, well, Lupin is not Lupin. Honestly, Lupin really isn't crucial enough a character to make this a burning question (as opposed to a Ron not being Ron situation). This lack of relevance is the problem with many theories. The Snape-is-a-Vampire theory. So what if he were a vampire? What would that mean? That...he's going to bite Harry? That...he'll pick the garlic out of his food at dinner? Take a different theory, say, was X at Godric's Hollow the night the Potters were killed? This is relevant because if X was there, X may know some hithero unknown important detail. This is the difference. So, when it comes down to the question of when a theory is total Hogwarts hogwash or not, the question is, what difference would this make? So, just keep that in mind whilst you spin your Harry Potter webs.
While we're on this topic, here's another thing: Dumbledore. He is dead. Dead as a doornail. First, his portrait is in his office. Second, everyone knew he was going to die at some point because this is just what happens to wise old wizards. Unfortunately, Dumbledore was not an Istar so he can't come back. Third, I think it would be mean trick for JK Rowling to pull, after all the emotional upheaval at the end of HBP and I don't think she'd do that. There was so much Dumbledore closure at the end he doesn't need to come back (this is one reason why it would be nice to see Mr. Black again...and to get more Mr. Oldman in the movies). That reminds me of a really funny part in POA about Humphrey Boggart. Hahahahaha.
Anyway, so in conclusion: Most theories are pointless. Dumbledore is dead. And, finally, while the debate is interesting, it's kind of pointless to debate the What-are-Snape's-Loyalties? issue because JK Rowling has made his character so inscrutable. He could go either way or a different way altogether and it wouldn't be inconsistent. Oh, and as for those Snape-died-not-Dumbledore theories, whatever.
So anyway, what this all means is, what are people going to talk about after Book 7 is out? Are people going to debate for the next ten years about how many kids Harry and Hermione had afterwards? Why Ginny had to die a painful death at the hands of an irate Prof. Sprout? The true meaning of the words "Never tickle a sleeping dragon"? Maybe it will all die down except for the fanatics until 100 years from now when Peter Jackson emerges from carbon freezing and remakes all 7 movies.
And just for the record, while I don't buy the Polyjuiced Peter theory, I am convinced there is a character that is not who they seem (possibly for the whole series). Either polyjuiced, a metamorphmagus, an animagus or something else remains to be seen but I'm sure of it. Maybe it's even R.A.B. Wouldn't that be intriguing? And relevant. I am so wize.