I understand the "Just be glad it's getting more fans" sentiment, but it's a psychological fact that people resent it when the things they like become popular. It's human nature, really.
I admit that I myself have no patience for people who only see the movie of the musical. Now, I understand the "Can't afford to go to Broadway" argument. Heck, I'm stuck in the Midwest; I've never even been to NYC. But there are tours which aren't horrendously expensive, and, heck, Sweeney Todd is available on DVD and everything. I don't like how people only acknowledge that a show exists because it's now a movie.
Also, if the movie is bad, people will automatically associate that with the show. For instance, I cannot even begin to name the number of people who have said "The musical Evita sucks; I saw the movie and it was boring." And I'm like "The stage show is SO DIFFERENT!" It's frustrating.
I also however agree with BSoBW2 who said that there are a lot of new Sweeney fans who are just into the revival and it's quite aggravating. Not that I'm an old Sweeney fan, mind you
I actually though wouldn't mind Sweeney shirts at Hot Topic. I'd probably buy one.

Though I already have one from the revival which I can safely say tends to freak people out.
Jimmy, what are you doing here in the middle of the night? It's almost 9 PM!