We're not talking about webmail here, but direct access by HTTP request, either from a website or a desktop/mobile client. Some other relevant links for this:

http://blog.gaborcselle.com/2010/02/how-to-replace-imap.html

http://blog.webhooks.org/2009/02/13/restful-email-over-http/

HTTP has had about 20 years of use, development, testing - culminating in SPDY now, which could get even 40% faster in some cases. I'm not sure I can think of a good reason not to make the switch.