Schilling and Sheffield's contracts should in no way be understood as the norm. Generally speaking, a player is at a distinct disadvantage when negotiating his own contract. Moreover, player's who do so tend to take the negotiations a lot more personally.
It is not easy for a top ballplayer to go back-and-forth with a team GM who is trying to convince the particular athlete he is not worth his asking price, or that he can go fetch another ballplayer at a cheaper price.
To go through that, and then eventually have to play for the same group can be burdensome for the player. The agent's presence minimizes such an effect.
In Schilling and Sheffield's cases, the reason they received a raise, contrary to Gonzalez and Alomar, goes far beyond market and demand.
In both situations, top management was under pressure to get both deals done. Hence, their (the players) had much more leverage than if they were regular free agents.
Also, both are still star players who can be important individuals both in the clubhouse and on the field. To guys like Steinbrenner and Lucchino/Epstein, signing these players represents a) wins, b) increased gate revenues, and c) excitement in their community, which, in turn, brings in more outside revenue such as sponsorships.
I doubt their will come a time in which agents become expendable.
Just remember this, agents are like doctors and lawyers (bare with me). Everyone criticizes all lawyers and doctors, except their own. The one they visit. The one they trust. To most, all lawyers and doctors are scumbags, "except Dr./Attorney ___, he's my Dr./Attorney, and he's not like that."
What I'm getting at is that an agent is supposed to watch out for his client's interest, and the general perception of us as greedy and sellfish is just that, a perception that not necessarily reflects the reality of it.
Said reality is best viewed from an individual ballplayer's perspective.
FM