I'm not sure the movie would have done better with a pg-13 rating. I have never heard of The Watchmen until the previews were on television. I'm sure many others haven't heard of it either before the previews.
Most rated R movies would of been just fine with a PG-13 rating. Considering that most of the films just add a lot of unneccesary violence/sexuality that is not related to the story line. I do believe that a super hero movie could be made with a R rating and do extremely well if the material isn't just added for the shock factor but actually flows with the story and is needed. Also, it would help to make it a well known superhero.
I think that a lot of people do disagree with the studios decisions on movies and would/could make a better superhero movie. There is a difference though, These particular idiots (The Studios) have a lot of money to produce these movies. Which, we do not.
Just my two cents though.