Well, to be fair, I wouldn't be okay with only 30% of the revenue, because I write very detailed scripts, and as both a writer and artist, I would say that I probably spend just as much time writing or preparing plot details as I do drawing.
Maybe other people just throw a script together, but I don't. I'm sure there are a lot of artists that argue that writing is worth less than artistic ability, but as someone who does both, I adamantly disagree.
I mean, obviously, just my personal opinion, but in the end, that's all that the discussion is, a bunch of people throwing opinions back and forth, since there's no way to prove the validity of either argument.
Something to consider, and this is true of any profession: a writer who is okay with 30% revenue probably doesn't take their job very seriously. If someone doesn't have faith enough in their own craft to think that it's worth a decent wage, perhaps it... isn't.