Sirius Black joins the website for the sole purpose of playing it.
Sirius Black becomes a LM for the game.
Sirius Black helps co-found another RPG several months later.
Sirius Black joins several content staff branches
Sirius Black becomes a citizen.
Sirius Black starts participating in the Curia
Forgive me for using my own client as an example, but I think this clearly demonstrates that RPG play does not solely benefit the RPG forum. It attracts new members which benefits the whole site and clearly keeps members around long enough for them to make great and recognizable contributions to the forum as a whole.