I don't think choosing aisle-aisle is rude, but it is a bit selfish.

Most people are more comfortable being in close proximity to someone they know than to someone they don't. If you are traveling with someone you know, it is more comfortable for one of you to sit in the middle seat, with your companion on one side, than someone else sitting there with strangers on two sides. The more couples who start taking aisle-aisle, the more people will have to sit in middle seats between two strangers.

Of course, this being Flyertalk, where people don't care about anyone besides themselves, I don't anticipate much support, but that is my opinion.

Oh, and contrary to what someone posted, flying Southwest is a good way of avoiding this situation, provided you check in early enough and have a good enough selection of seats when you get on.