I too only use webmail, but have a variety of accounts. I've never cared for downloading my email to computer. Reason being that I don't like the idea of having duplicate copies of my email all over the place or being limited to one machine. I switched to primarily using Gmail last year, but made a mistake and signed up for multiple accounts. I used to have to do this when providers only offered a small amount of storage and without think did it at Gmail. Other than that mistake I'm very happy with my webmail decision.
Most of my friends use AIM or YIM. I try to avoid AOL products like the plague and never thought about using their webmail, but I do have a Yahoo accounts. I use GAIM to connect to all my IM services but not for IRC.