I think it really does help them, because I strongly believe that no one would be willing to pay other people for nothing, especially nowadays, nothing really comes for free. 
You said it correct. Advertising is also an investment for these ICO's and there's no signature campaign that will exist if it's not effective to the first ones who tried it out. We are on the main forum of bitcoin which is also including other crypto's so advertising through signatures is also a good one to promote their project. If there's no effect to the early advertisers, they won't recommend of doing it for others. But this is the service that they have paid for, it really works.