It is not so simple. There were, and most likely still are, huge campaigns that recruit hundreds of users while there may be not so many good posters, thus the campaign managers which are chosen to run these campaigns might not have any other option but to accept almost anyone who knows how to sign up for a campaign and not make a dozen mistakes therewith...
Forum does have a lot of good quality posters.Just that campaign managers are too quick recruiting the shitty ones.I'm not saying add only members who are as good as DannyHamilton but at least a two sentenced descent post which actually adds something to the topic and not repeats the same posted in 100 comments above is expected.
On the other hand, the services themselves may be more interested in the sheer exposure their ads get than in the quality of posts
Wouldn't they get the actual exposure if the person wearing the signature seems knowledgeable and fluent?