Seems like an interesting proposition and makes sense overall. I can think of a few issues however.

1. You always need access to email to get access to a website. This means that for whatever reason you don't have email access, you are locked out of all sites.

2. While it works from a security standpoint, you still add overhead to the users task. Users have to go between website and email and back to website just to log in.

3. slow connections might make the task even more cumbersome since you now add the delay of checking emails just in addition to logging into the website.