The Aquarium is a necessary evil. For a topic-specific-forum (saltwater, planes, cars, etc) to have any staying-power and last, there has to be a successful 'off-topic' area. The truth of it is that not everyone can live in a certain field 24/7, so instead of leaving the site, they just go to the off-topic area. Then because they're still on the site, they hopefully migrate back and forth. It's also a way for members no longer active in the hobby to keep active.
I will agree with you peef, and say that around here, the Aquarium is unusally large/popular as a percentage of the whole forum, then other message boards. Not that it's necessary good or bad.