What are Gmail’s strong points? The most obvious is the search function. The e-mails that I find myself searching for most often are mailing list archives. Ergo, Gmail would be good for storing mailing list mail, which usually ends up in the trash anyhow. Any other mail, I agree, benefits little from residing in a Gmail inbox. The cost of switching e-mail addresses simply does not outweigh the benefits.
I’m 100% with you on the XUL Gmail client.