There's a few pros and cons for this.
The pros are that members areas likely won't be as filled as F2P, and if there ends up being bots, there will be extremely few of them in members areas. On another remake, one problem is that there's a lot of flax bots in Seers' Village, and that would be pretty much impossible to do with this. Also, fletching won't be the way most people make money when they first join, which makes it hard to wc because every training spot is crowded.
The cons are that it would be a bit annoying, and people who make pures would have to train other skills. And some people would probably leave, if they think that they have to pay for members.