For an international flight, you won't need it, as in my experiences with American, they have good service on the long hauls, so it really isn't needed. And most airlines(I think including American) have a snack table by the galleys with snacks and munchies if you get hungry while there isn't any meal service at the time.
However on United States domestic flights(like AerLingus said, some lasting up to 6 hrs). I wouldn't call it rude. As long as you don't oderize the cabin, it's okay in my opinion.
On most flights, I bring food such as munchies and candy. On two Southwest flights on my way to and from Lacrosse Camp, I brought a carry-on filled with candy. Luckily, no one saw the candy
Give me a break, I created this username when I was a kid...