The sites paying to get slots certainly need the advertising, that's why they pay for the slots.
Does the forum need the revenue from paid advertising, I'll normally have said yes, but considering theymos is considering shutting it down, then maybe not.

While paid ad slots may not be that necessary, the incentives from signature campaigns goes a long way to keep discussions alive on the forum. Many of the discussions are spam, but even genuine, constructive conversations would slow up if there were no signature campaigns.