When I first got a computer (2001) I did a lot of email, including a lot of Yahoo Groups mail. Since I was on dial-up, and we only had one phone line, I used an offline email client, Outlook Express for reading & writing. Soon I started trying out just about every other client out there. Premium ones (like the Bat) I only used for a trial period. I tested loads of them, but usually came back to the simplicity and (for me) reliability of OE. I would set up email accounts to POP and then "leave mail on server." Later I'd go to the webmail interface and move messages to folders. No big deal. And I found this method easier than IMAP.
When we got cable internet I began to use webmail more exclusively, though I have enjoyed all the functionality in Outlook 2007, which I only use for an offline back-up. For reading and writing I have pretty much moved strictly to webmail.
But I have a very short email attention span, so I'm constantly using my different webmail accounts for reading, writing and sending.