Current rule is
"Affiliate Advertising: The Marketplace is intended for advertising products and services owned or operated by forum members. Affiliate or referral "links" are not appropriate anywhere on the forum."
As a prefance, limiting affiliate links is not unique to this forum. I can name at least a dozen other forums comparable in size to or larger than NamePros that have similar rules (sitepoint, webhostingtalk)
I'll break it down into three main areas,
1. Marketplace forums Our marketplace forums are intended for advertising products or services you own or operate and not for affiliate links. The logic behind this one is the most simple. Everyone is or can be a member of an affiliate program (or even dozens of affiliate programs). If these areas were filled with threads advertising affiliate links, it would not take long for the entire area to be overrun by them. And imagine if we had dozens of threads even promoting the same program by different members. Not a good idea there.
2. Posts/Replys - Permitting affiliate links here may lead to recommendation of services solely because the member has an affilaite link they want to promote. If someone asks for hosting services, are they going to get honest recommendations or a bunch of replies from affliiates of the highest paying hosting programs. Additionally, it wouldn't take much for an affiliate to search all threads on our site and find dozens of threads where they can interject their affiliate link into the discussion.
3. Signatures - This is less obvious, but the opportunity for abuse is similar to the last, where an affilaite may take the opportunity to get involved in discussions only to promote their affilaite link. For instance a domainhop affilaite participating in discussions about domain parking to discuss domainhop for the sake of getting their affilaite link in the thread. Now consider if that affilaite program was so lucritive that many domainhop affilaites started doing exactly that. What effect would that have on the discussions? Would better programs without affiliate programs receive less attention?
Like mentioned previously, its also to curtail spamming. I've seen many affiliate programs and tip articles encourage the practice of joining forums for the sole purpose of promoting your affiiliate links in posts and signatures.
That being said, I don't feel affilaite marketing in signatures is inherently wrong. It is a perfectly fine marketing method when its not abused. Obviously we can't allow affiliate links in threads and posts. The signature rule has never been openly discussed, so I am willing consider all sides of that discussion.