no. steemit users don't need to buy more steem. but i will grant you that yeah, thats the current vehicle that brings fiat money in and out of the system but it not ultimately how its going to work.
Steem is a way to get fiat money in and out of the system at the beginning for early investors and adopters. But if this works the way I think they designed it to work, i don't think youll even need it at all except to fund a reserve.