someone have 5-10 accounts and he have no time care about what he is advertising, but keeping post. to get paid.
and for this situation . adv on this forum is really expensive.
am I right?
Probably. Keep in mind however that there are still a great number of actual users (not account farmers or spammers) to see your ad.
most sig campaigns should have a few basic rules
1. some method of communicating abuse of spammers to the sig campaign manager to void their reward
Most signature campaigns do have this. The problem comes that certain campaigns don't, which are usually the ones with the lowest quality members.
2. highlighting the content of peoples posts should require relevance to the topic and an actual meaningful reply
This could be difficult as what is 'meaningful' can differ due to personal interpretation. While I do agree with you, the result of this would likely differ per campaign (therefore making some more attractive to spammers than others).
3. not spamming a particular category or topic
I can't say I agree with this. I post in the topics that I do because it is what I am most knowledgeable with. I think quality posts in said sections would be better than posts with little value in others.
In my opinion, a lot of "legitimate" users would come to post sporadically if they weren't paid by advertisers
Absolutely. In fact, I expect that a few could start to re-emerge due to the absence of 3/4 of spammers. Not everyone here posts solely for signature payments, just a vocal group.