We use Post Affiliate Pro because WHMCS's affiliate module was kind of limited.
We haven't really done any advertising for our affiliate program though, so I can't honestly say having one is helpful or not yet because we only have a handful of affiliates so far.
All in all, I would say adding an affiliate program certainly can't be considered a bad idea. It's basically a means of advertisting that doesn't cost you anything unless you are selling your services.