The forum is attractive now due to the traffic it pulls. Campaigns looking to get people talking about their project would consider advertising here, cause they are guaranteed to reach a large audience, if signature camoaoniarr removed, majority of that traffic would disappear over the coming weeks, greatly reducing how viable the forum is for running a spam campaign.
I've been on forums without the reach or traffic bitcointalk offers, and spam is barely a problem there.

100% might be a reach, but a huge chunk of spam would be cleared up without signature campaigns.