What a weird double-standard. A feature of email (that you can have several, and you can change which one you use) is a failure, and a failure of Facebook (that it's one site, forever) is a feature?
We're all familiar with ways to migrate from one email address to another. If/when you leave Facebook/it disappears, how will you migrate your identity then?
My Facebook account is an identity first and foremost. People do not typically have multiple Facebook accounts (a TOS violation). If that account goes away, that identity goes away.
You might make an argument that websites should allow you to aggregate multiple identities (Facebook, Google+, MS, Yahoo, etc) into a single account, or that there should be some sort of an identity provider that creates an aggregate identity across all those services. To an extent some of these websites are already doing something like this peer-to-peer - Facebook is an OpenID consumer, for example. Maybe sometime in the future this will be a big issue. But right now it isn't, and email-as-identity is already an annoying problem.
Now imagine 5 years ago you said "My Friendster account is an identity first and foremost."
On the other hand, email is already known to be an unstable key for identity. And the "market" for identity providers is a lot more mature than it was 5 years ago. Besides, what if Friendster had established itself as a public identity provider 5 years ago? Maybe we would be using Friendster instead of Facebook today. Who knows.
Would be happier if there was only one company that provided email service and you were only allowed to have one address? That's essentially the situation with Facebook.
You're free to apply whatever constraints you like to your use of email. An email address is as unstable an identity as you make it.
Two close friends of mine were the #2 and #4 employees of Friendster. I seriously considered becoming #3. I had a pretty good outsider's view of the early years.
Yeah, they screwed up the scaling pretty badly. But even worse they screwed up the business - after you set up your profile and looked around, there wasn't much more to do (unless you were single and looking). People were already using Friendster as an identity (emailing links as a "you mean this person, right?"). Maybe if they'd opened an API and enabled third-party apps, they could have maintained this position. It's a big "what if" but it can't be dismissed outright.