1. If your provider goes down, you lose mail.
2. If you are conversing with people who are using an insecure mailer, such as gmail, Yahoo, etc (which is probably > 99.9% of all e-mail users), your e-mail is still accessible to the NSA, or to some Fortune 100 advertising company.
3. It's only a matter of time before the "big dogs" in email abuse the position and decide who is and isn't allowed to send/receive email outside of their little oligarchy, either on their own or at the behest of governments.
Like so much else that has been corrupted, we need to scratch the current architecture as too insecure, and build something truly secure for the future. This isn't in the interests of the Googles of the world, and it's actively in the worst interests of the NSA/FBI/CIA, so it's probably the right thing to do.