I think the 10.95 monthly plan is only available after the first year now. Two things to consider, first they practically give away that first year anyway making it unlikely anyone would ever want to go monthly at that point anyway. Second, when you could sign up on a monthly plan there was a $49.00 setup fee for monthly, again making most people say why would I want to do that?
Now comes the complicating factor… The affiliate program, when you can talk someone into signing up using your affiliate ID instead of a dreamhost promo they will most likely pay more. Sometimes in the case of a developer/customer relationship this might happen. However for those just shopping for a webhost, the dreamhost promo is always a better deal than an affiliate can create.