Interesting point. Someone with a long, detailed and well written backstory is more likely to be seen as a competent roleplayer than one who's got little to show in terms of fleshy backstory. But like you say, that isn't to say they aren't good roleplayers, just that they don't like writing that much.
I think to be a good roleplayer you need to be able to lay at least a groundstory, but also have the imagination to carry it through into the Disco universe and evolve your character and his story through your actions out there. What's the point in creating a brilliant backstory just to have some character that's doing missions for the Kusari Police when he's a patriotic Bretonian or something? I think the key is to incorporate both good roleplay on the forums and ingame.
A cynic is a man who, when he smells flowers, looks around for a coffin.
~H. L. Mencken