I completely agree and I've said the same thing in the past.
Signature campaigns lead to more ad views which translates into more revenue for theymos. Ban the signature campaigns and views (aswell as posts) will drop (undoubtably by a huge amount) and ad revenue will suffer.
So the most important thing here is profit? When did this forum become a corporation?
Another alternative, keep signatures and ban signature campaigns with strict punishments if someone does deals in private.