They aren't overrated in this capacity. Wildhack [and any AD] would need the firm because what they do isn't just about helping identify candidates -- they also do a lot of vetting of interest behind the scenes, prior to that list taking shape.
So, just for example, let's just say that Wildhack targeted Billy Donovan [I'm not saying he will], but they are concerned that he might turn them down because he's in the NBA, paid more than SU wants to offer, etc. The search firm would have representatives who would have that conversation initially with Donovan, gauge whether there is mutual interest, and then proceed [or not] accordingly. It helps both sides save face. SU never formally offered or even had a conversation, so they aren't turned down -- and Donovan avoids unwanted distractions that might leak out in the press, or cause any professional embarrassment.
Again, based upon your post, not sure that you have a good grasp of their role. Wildhack isn't just going to bring them in to compile a list of names while he twittles his thumbs -- they are far more active in the process than that.