Whether an agent is needed depends upon each player and family. Finding the right agent is tough - and depends upon a research, recommendations, and luck.
I understand the position that a player earning 1100/mo doesn't need an agent. I'll look at the other side of that.
My son earned that princely sum and did pay his agent the 4%. For that he got annually multiple pairs of cleats (players need to buy their own), gloves (ditto), clothes, access to a sports psychologist, annual retreats where he could hang with other players up the chain, seminars on taxes, investments, health, nutrition, etc. Got this for his $260+\- per year - not a bad deal. If he didn't get the items (gloves, cleats) he/us would have purchased it.
In conversations I had with an asst GM (now a manager), he hated agents calling him about released players. He wanted the player to make the approach (said every talk with an agent looking to have a player signed wound up being told what a crap team and players the organization had and that's why the player should be signed).
I don't believe an agent can move a kid's position on a draft board based upon talent; an agent can move a kid based upon bonus demands. I have seen agents get scouts to watch individual workouts as well as watch games - just like a competent personal coach or HS or college coach could also do. I have seen released players get signed out of the workouts. My point isn't that the agent accomplished what no one else could; just that he can be a piece of the puzzle under the right circumstances.
The first time we went through the draft (HS) no agent was needed. W and I were pretty firmly in favor of college and his personal coach had a great deal of familiarity with the draft (with the bias strongly to going pro from HS). We were able to come up with a number and deal with the clubs - and he wasn't going to be a single digit pick.
The second time, he had given up on pro baseball after three disastrous collegiate seasons, but the release of that pressure somehow allowed it all to come together as a senior. After getting some crazy number of scouts at the latter season games (see below), he felt he wanted some professional help and went with an agent his baseball buds had. It was cheap and it worked out. As a senior budget pick he had limited negotiating power, but did manage to snag a decent bonus as he was contacted by various clubs during the draft (agent helped him to not price out of market).
When S was released, agent immediately called and gave him emotional support (getting fired from the first real job is hard), and offered to find him another club. S had had enough; the game wasn't any fun and he wound up turning down multiple offers to sign. But the agent did help - son was the example of an agent investing in a whiff of promise and actually delivering a lot more then the agent received.
There is no right and wrong; only educated consumers can make that decision.