If someone does not have his signature campaign, it does not mean that he is not promoted in other places and other ways.
I agree that the ones not running signature campaigns might be promoting their services with the help of advertising on other sites, but I still believe that other promotions along with a signature campaign could give way more exposure to a platform.
Especially, to mixers I believe a signature campaign could be quite helpful as most of the users try to search for mixers on forums, and after banning of mixers on Bitcointalk, the only reliable forum that allows mixer advertising is Altcoinstalks.
Most good mixers will surely try their best to advertise on this forum as they know that with advertising here or running their signature campaign on this forum, they could get attention of the users who search for reliable mixers.