Yes it deserved the PG-13 rating. Easily. This is the problem. If Lucas gets to slap on the PG rating then ALL of the parents in the world think it will be just like all the other films...safe. This one is not as safer as the other 5. At least not for little kids that generally go to PG films.
This one has the hero going bad, getting his legs and arms lopped off, burnt alive, painful screams, hero's wife choked almost to death, while pregnant, some scary music, deformed faces, kids being killed by the hero, etc. etc. As a whole it's too much for some kids and there needed to be a warning. If there had not been could you imagine the outcry. It would have been bad news.
Most kids can take it I realized that. However, the second showing featured not one, not two, but three different children crying at various points during the film. Not just at the burning of Anakin either.
One kid lost it while seeing Kenobi and Yoda looking at the dead padawans. Another during Palpatine's face getting deformed. Another kid was escorted by his parents during the Padme's pregnancy screams. I think one of the same kids mentioned allready was murmuring or crying when Anakin burns.