I think that making it PG-13 is interesting, but it might have an immidiate boost to sales that most weren't expecting. I think some people (like Gremlin
) will see that it has PG-13, think that that will make it cooler because it is more violent, and want to see it.
Thats just my opinion, but so many people want to see the bloody movies and the more action packed movies. Now, it might ruin the running of a clean trilogy, but who cares for the sake of money?