how has the Advent of signature campaign affected the forum? Is it positively or negatively?
If it's done right it should affect the Bitcoin economy positively, and probably signature campaigns along with the services board are the thing that made Bitcoin strong today since it was the forerunner of "cryptocurrency" trades.
So it's not only about the posts but the economy itself in the beginning (based on my humble observation).
Should there cease to be signature campaign on the forum, what's likely the effect it will have on an average member of the forum?
If it stops today then this forum would lose most of its activity since most members participate in the economy in various ways, such as designers, campaign managers, signature participants, etc. Basically, the forum needs content and only an extremely small percentage of members will provide it for free. Without an incentive, maybe people will still log in but as a silent reader, and if this continues, economic activities along with the discussions will take place in other places eventually.