If you ban signature campaigns then account sales will probably drop to almost nothing but forum activty also drops, does that matter? i dont know.
You decrease the activity of the spammers, which is something that is good to see. Think about forum activity in the bigger picture for a second here. Those threads with hundreds of pages in Economics, Gambling, even Bitcoin Discussion are filled with one-liners and repetitious content that offers no substance to the forum. So what are we really missing out on?