I think whatever risk there is in the PM password setup is limited to damage that may be suffered by the PM website itself, and by the community as a whole that it serves.
You are an exception to the rule. For IM, I use sporte01. For yahoo messenger, i'm sporty3, but point is, 95% of the time, I use the same login all the time. Having worked for a community site company which ran 3 communities (about the same schema for each instance, marketed different) I found many MANY people to use the same login when they can. Same password, I can't say, since they were encrypted. But point is, your login tends to get reused, 'cause it's just as personal if not more personal as a password.
Heck,.. I actually get called sporty in real life.. so what else should have I used as a login?
Play that funky music white boy..