Yes, that's kinda what I'm talking about.
Many folk seem to be using PbP to try to replicate a table-top game environment, meanwhile they're not necessarily exploiting the uniqueness of having a whole forum chock full of players, many of whom are hungry to game (even if only for a cameo!). So when you have 10 people trying to sign up for a 4-6 player game, I'd approach the extras and find places they can come in and whip butt on the PCs, or be a key contact or something.... their interaction might be a couple weeks, on & off, here & there, but you're not saying "No, you're BLEEP outta luck, my game's full." and you get to size their playing ability up for the next time you run a game, too.
Even as players, we might approach the GM of a particularly active game and ask if there's any roles they'd like them to handle. Fixer? Johnson? Lead contact? Baddie? Goon squad leader? Damsel in distress? Dependent kid? Spouse? heh.... the possibilities are endless. Then again, I'm all for the roleplay -- not the roll-play. Heck, I can roll dice and build tough elephant-gun-slinging characters, etc. but to me the important part is being someone else in another dimension for a while.
I figure the GM has so much to juggle, why not hand a few guys over to someone else for a while. A few PMs later, you have a baddie who really has a life of their own -- as long as it sticks to their background & and general MO/motivation guidelines.