IMHO, I don't find this necessary for a few reasons:
1) Identity. Pretty much explains it if you don't fancy remembering different login combos.
2) There are websites linking usernames to IDs so that's half of the hacking challenge gone.
3) You can't do much with usernames, but not with emails or passwords which are rich in information.

But like what hd mentioned above, I agree you shouldn't use your real name.