While I'm a strong believer that signature campaigns enrich this forum's quality and that “it keeps it alive”
Signature campaigns are part of promotions, advertisements of companies in the forum. They together help to bring more traffics for the forum but without them, the forum won't be a dead one. There are times the forum does not have signature and/ or avatar and it still survives so well.
Once I read the replies of a thread, I'll ignore most of the users that participate on a low-paying signature campaign, by the thought that they are
shit posters. But, that may not be true. I observe
a form of prejudice. I'll actually take a closer look to a post made by a high-quality signature campaign participant rather than a newbie.
It is your bias and any of us can have too (at somewhat extent).
I disagree if you say the quality of specific posts is decided by the campaigns they are participating in. A user who are in low-paid signature campaign or bounty can still make good or outstanding posts. It can be expanded to ranks, a newbie can make outstanding post as same as or even better than a Legendary member.
Generally, the average quality of post would be different (significantly) between participants of high-paid and low-paid signature campaigns. It would be more accurate if you generalize it from a total sample size of participants.
A user may have thousands of merits, but he/she may create low quality posts. Someone with 4-digits post count and 2-digits merit count would be quickly rejected into your mind, either because he/she may have promoted failed/scam tokens or made shitty posts for pennies.
It is related to posting style. If you spend 2 or 4 years to rank up, with due diligent efforts for your posts, your writing skills should be sharpened and better. Over years, it will become your habit to make good contributions with above-average quality posts. However, you always can make shit posts.