I am not that anti-chatting but am not a huge fan of the "who's online" thing that pops up every min. It's rather distracting. The "disable" button doesn't do anything expect reload the page.
If you click the little cog-wheel, you can select 'disable pop-up notifications' and mark yourself offline, so you won't get random popups and people might not see you online at all.
I'm with JoJo, I *HATE* chat systems and disable them wherever I encounter them. I'm very happy to receive messages, email-style, and reply when I have time to compose and refine a proper response, but I'm a "single focus" person, so if someone tries to chat to me it kills anything else I might be doing (like trying to edit scenes for release) completely dead until the chat is over.
Henry's advice is the way I've set mine, so it'll always show me as off-line, even when I'm here. Happy to receive PMs and emails, and reply to forum threads, but I don't do chat.