A few...
I register the name "Lukie" on other websites so people don't take it. I don't actually use those sites.
It's a credibility thing. I strongly (emphasis: STRONGLY) believe that people should be given credit where it is due. Conversely, people should not be given credit where it isn't due.
Honestly, if someone else registered any nickname I use, I'd probably snap. Especially if that person was "less than savoury" or just downright creepy. I feel their bad image would rub off onto me...