Anyway, even if users become part of those signature campaigns (don't take it personally, I simply try to describe how it works) they influence the others. Moreover, if a beginner comes to this forum and see a Legend member or something like this, the chances that this newcomer create an account via the link from the signature are higher.
Why we wear signature on the bitcointalk forum, do you know? We wear signature campaign as a promoter of the project/site on this forum. The people might be interested for that signature but referral link not allowed so there is no chance to that newcomer will create an account
Also, when it comes to legitimacy then im bit assured that signatures that be proven out would really be somewhat that legit since the community wont really be blind on not to notice those shady stuffs around.