At the same time, as a user, you need to consider your own e-mail setup. You should not use your ISP-provided e-mail address whatsoever, there is absolutely no reason to tie your contact to an account you may not keep.
Likewise, you should not be using your school or work-provided addresses for something that you expect to use beyond going to school or having that job.
If you're changing your e-mail address frequently for personal reasons (eg, someone harassing you?), then you should actually consider creating an account you use only for authentication, and a separate one that is your public-facing persona.
Having separate addresses for your contact and sign-ups is actually not a bad idea anyway. It actually makes it a bit harder for someone to break into your account somewhere (because they don't necessarily know what address you used), and you get the freedom to change/abandon your public e-mail address if necessary.
One argument against emails-as-names is that some people change email addresses frequently - unless you have some way of accounting for that (and you definitely wouldn't use the username as the contact field), the username will eventually go stale for some users.