The way the amatuer draft is set up now with assigned slot money, I wouldn't think anyone would really need an agent after the first few rounds.
There is value in having an agent. Most agents take a fee from your signing bonus, if the agent has done nothing in that process, then his fee may or may not be worth it, unless there is value after the agreement. But reality is most do, as much as the NCAA says there are things you cant do, they are done, you just need to feel comfortable with that person and understand the trust factor.
Most good agents have good relationships with companies, pitchers get gloves and hitters get bats, and then there is all the gear that one may need, every season without the expense. The agent will never see another penny from the player until he signs or if he signs a milb contract. Also you may need that agent for arbitration. So once you put up that fee, how soon do you want to end that relationship or keep it for awhile? What happens if you pay someone a fee and then released a few years later. These are all things that one has to consider.
One must really do their homework and decide if they feel it is worth it at the time they are drafted. Just keep in mind that for a later sign it is not necessary.
There was an a discussion on the ask the scout forum about a junior redshirt entering his senior year having an adviser. While I understand they may have needed the advice, a junior redshirt entering senior year probably isn't going to sign for much or high regardless and doesn't need an agent at that point. Sometimes players just take on adviser/agents because they feel they wont get drafted without one, not true.