Actually if they are concerned, they should stop and think about what the big movies/books were when they were kids, and how kids played/acted back then. This is something that is repeated every generation.
When I was a kid, it was cowboys and indians, cops and robbers, pirates, knights and wizards, soldiers, etc. Funny how little changes (cowboys and indians have probably went away and been replaced by space/sci-fi characters, but the others are in full force).
I will admit that some kids probably have problems with roleplaying, but their problems existed long before they ever donned a robe, picked up a lightsaber, strapped on a cap-gun, put on a toy helmet, or picked up a toy shield and sword.