What happens is this; if you stay logged in perennially, on meebo, it'll still be a valid account there and you can talk to people. But when they log out, you loose access to talking to them. And, when you log out, that's it gone.
Leave as in delete your account? Well, first of all that would suck =/.
I don't think you can access meebo unless you're logged into GAG. However, what you could do is just not delete the account and log in only to access meebo. You effectively leave by not answering/asking questions, but you'd still be around to talk to people on occassion.