Sometimes I'm horrified by the films to which small children are taken - you wonder just what sense they're making of it, or wince as they whimper in fear. If I go to a G-rated film, I expect a kiddy crowd and the noise/popcorn/rustly unsettledness of it all. If it's M or MA, you hope not to have children running in and out, or clearly upset, or clearly uninterested in the screen action. But sometimes you wonder if a rating, focusing on a small part of a film, might deter a worthwhile experience for medium-sized or older children.
There's an interesting article about this in the New York Times: Take the Kids, and Don't Feel Guilty.