I kinda disagree with that. Situation was maybe like that ~2 years ago (I remember very good members fighting to get into lowest paid Bitcoin signature campaigns) due low amount of sig campaigns but situation now is completely different and it's a sellers market. There are way more spots in Bitcoin signature campaigns than there are decent members interested, resulting in managers lowering the bar as they have fill up the campaign.
I don't know how the situation was 2 years ago but maybe what you said about the current situation is true. There are places in certain campaigns filled with low-quality posters where I believe they don't have a contribution worthy of being considered a quality poster.
I don't think it's bad for marketing, but probably not very good for forums so this warning should be taken as a precaution by moderators.
Signature campaigns should be a way to reward users for quality contributions and only those that do should be paid, but at the moment it has become a way for many campaign operators to lazily and cheaply advertise their business by paying greedy users to spam whatever unsubstantial rubbish they can be bothered putting the minimal amount of effort in to and this will no longer be tolerated.