The main reason (in my opinion) that Django has an authentication component is because it has an admin section. That requires authentication. The admin section is something to be jealous of because it's a lot harder to duplicate. Creating a user model isn't that hard. While there have been Rails projects trying to implement an admin system as nice as Django's, they aren't as nice and clean as I'd like. And that's a lot more complex than a simple User model.
And yes, I'm aware that one can do lots of things to extend the Django User model. Examples: while the User model doesn't require an email, you could have the form you build require an email; in Django 1.2, you can have "@" and other email characters in usernames and then just reference the username attribute rather than the email attribute when you want the email; in your controller/view, you could first search for the user by email and, if found, grab the username from that object to pass to the authenticate method. It's more that a User model isn't such an incredibly complex piece of code and I find that different sites often want slightly different things that make it just easier to make one's own.
TL;DR: Be jealous of the admin section, not the authentication system.