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Reyes has the speed to create offense, but how long will it last?
When looking at baseball history, we see numerous cases of players losing a step in their early thirties. For most players, this isn't a big deal, but for a player that builds their game around speed it can be deadly.
In the 1980s, numerous players were speed players. We all remember Vince Coleman from the Cardinals and Mets. When he turned 29 (Reyes will be 29 next season), he became a marginal player. Granted, Reyes is a better hitter than Coleman was, but speed is so integral in his game.
A better comparison may be Juan Pierre of the White Sox. In his five best seasons, he played every game and had 10.2 offensive WAR. In the four seasons since, he has 1.4 offensive WAR. He has been more valuable defensively since moving to left field, but left field is a premium offensive position.
Reyes has 22.9 offensive WAR in his last five full seasons (excluding 2009), so again, he is better than Juan Pierre even when Pierre was at his best. That's not the point. The point is that Pierre's value has significantly diminished since he lost a step. Even if Reyes is half the player in the next five years, he will be vastly overvalued.