I believe running a Signature Campaign here in the forum is much cheaper than advertising in google adsense or other advertisement company that's why most of the businesses are choosing to run a campaign instead.
Not really!Advertising on google or any other ad agency is much cheaper than signature campaigns.Many campaigns spend more than 1 btc a week to pay their participants and yet the choose to continue doing the same.
Reason is simple : You get the niche crowd,the type of crowed you exactly need to advertise your product with.There is no other place on internet but bitcointalk where you get the most bitcoiners from all over the world.
Advertising signature campaign was not a waste of money actually, it is very helpful and very effective promotion, and sometimes good background design can give attraction for the community to click it. because if this things was not a good idea, I think signature campaign here will no longer appear in the forum.
Theymos has stated it,the campaigns will continue even when the stuff is migrated to the newer forum.