Jim,
They absolutely play a vital role in the process, as they are your eyes and ears into each club while they advocate for your son during his senior year. Feedback is vital, and they are the means to feedback. Why? You will have a strategy, and that strategy will need to be adjusted at every turn based on that feedback. And by the way, they will also advise as to what you should be doing if you're not doing it.
Large or small, help or hurt? I strongly recommend that you not consider that question, rather, ask which agency fits your requirements. Define those requirements, interview those agents, examine their track records, then come to a decision relative to your requirements.