The point of using your email is that basically everyone has one and it is easy for them to remember. Of course, you can use a random ID, but that's hard to remember.
It's even more easy to remember my real name or date of birth but I don't use them as login options anywhere.
I don't want my email address to be associated with anything I don't want, especially with bitcoin or bitcoin exchanges... because you never really knows who really is saving and reading all that data.