I would have to disagree with this. Take the Wolverine movie for example. When it came out as PG-13 I knew it wasn't going to be as good as it should have been. It didn't suck, but could have been so much more.
First Conan movie. Rated R, kicked ass. Second one, PG-13, sucked.
I think this movie will probably be ok, but the PG-13 may limit the amound of horror in it. JMHO.
Wolverine and Conan the Destroyer weren't that great because they were rated PG-13, it was because the writing wasn't that good. More violence wouldn't have helped that...Unless that's all you care about.
Not seeing a movie simply because of its rating doesn't make sense. There are plenty of R rated horror movies that suck. I would rather see a well written PG-13 movie than a crappy R rated one any day.