I honestly do not see how it is rude, at all. I come here to talk about fish, ask for help or give help about *fish*. So why would I have to tell everyone on the forums that am quite interested in drawing and that I work with children? Or why does it matter if I live in England or Holland? I come to talk about fish, anything else I'd go to msn for or some other chat applet, heck ... even the fishforums chat would do! Talking about things not related to fish aren't even accepted unless you visit the general hobbies section, so really don't see any point in filling in my profile information.
The only thing that I can understand is filling in your location, but unless you're willing to specify exactly where you live this isn't going to be a lot of help to most people. And I don't recommend to anyone that they fill in exactly where they live, simply because you *do* get freaks on forums that will try their very best to track you down, and often manage.
As to why I haven't got the aquariums and fish section filled in, that's because it simply changes too often and with me not being all that frequent to the forums anymore I'd have to log in to my settings every time I'd visit to alter the fish I'm keeping. Not to mention that 8 tanks simply looks messy. If I need help and people need to know what fish I keep and in what size aquarium I'll be sure to mention it to them. If people have a general interest and ask me in a topic, I won't mind answering them. But putting it in the profile to give people, most likely, outdated information I don't find necessary.
Heck, I take it to the extreme and dont like people knowing I'm browsing a certain thread because I know some people do take offence if you read but don't respond. Especially in the picture sections of the forum people can get quite annoyed with it. For this reason I log in as an 'Anonymous User' as well. I do understand a lot of people find this over-reacting and being too much of a 'hidden person who simply shouldn't be online if you don't want to give out any info or people to see you'. Up to them, I simply prefer using the forums the way I do. Not giving out too much information and not being seen and questioned.