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.
FACEBOOK is at the top of that list. And they wont think twice about giving out users privacy details.
Btw, I think users need to know that therere a few things to consider before signing up with a username since you cant change it.
It also depends on how much privacy you really want, and if you want to be consistent, especially if you get to make friends here. I was Dabs on a gun forum (where almost every other person there carried daily), I was Dabs on some online massively multiplayer game (EverQuest) and was a mod on a forum dedicated to a specific user class of that game, and I'm Dabs on a few other forums about different topics (different topics, yeah, heh.)
Using the same username on different platforms only comes with a transfer of reputation and trust advantage if the user can fully prove theyre the same person. I wouldnt trust someone with Theymos as a username on a different platform. Therell have to be need to verify which makes no difference in general.