I searched the forum for this subject and received one hit from June of 2010.  I'm in a situation where the guy chasing me has players at 88%; whereas I have been resting mine off and on for the season and all of them are above 95%.  His players show no sign of having this catch up to them (one of them just won players of the week for hitting over .400 for the week).

I searched the dev chats and players guide, etc, and the only mention of the downside of position player fatigue was an increased chance of injury.  The forum post has a lot of guys saying they think it affects defense, but it was all so subjective.

Does anyone know why I should care if my stars are at 85% (despite the increased injury chance)?  'Cause I feel like a chump for resting my stars in the middle of the season.

It's logical to assume that decreased fatigue should lead to decreased performance.  The PotW award example could be a statistical anomoly.  Sounds like you're in the lead for either the division or a WC spot.  So whether or not you continue to rest your players depends on a couple of factors: (1) the size of your lead; (2) how many games left in the season; and (3) whether or not you've clinched a playoff spot or not and are just playing for your seed.

My thoughts on (3): Unless you're looking at the difference between a first round bye (1/2 seed), or a 3/4/5/6 seed, it doesn't really matter unless you're looking at team matchups.  Home field advantage (3/4 seed) is not that important unless you play in an extreme stadium, so resting your guys and settling for a wild-card (5/6 seed) may not be that big a deal.

Increased chance of injury is a pretty good reason.   Injured players don't play and, if the injury is bad enough, they can go from star to bench player while making the big bucks.

Due to the nature of the game, I don't think anyone can definitively say "Fatigued players produce less".

Posted by MikeT23 on 5/16/2011 10:25:00 AM (view original):
Increased chance of injury is a pretty good reason.   Injured players don't play and, if the injury is bad enough, they can go from star to bench player while making the big bucks.

Due to the nature of the game, I don't think anyone can definitively say "Fatigued players produce less".

0% Pitchers do. There is no reason to believe position players would be/are treated any different, the %change is usually just much smaller, i have no doubt that fatigued players have a lower chance of production than they would at 100%.

True.   I took a team in Foxx that had been abandoned for 62 games.    A few examples:

Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Yamid Valdes
Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Pascual Rios
Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Bartolo Cora

You don't have to be brilliant to see that they were much better pitchers at 100%.

Posted by tecwrg2 on 5/16/2011 10:18:00 AM (view original):
It's logical to assume that decreased fatigue should lead to decreased performance.  The PotW award example could be a statistical anomoly.  Sounds like you're in the lead for either the division or a WC spot.  So whether or not you continue to rest your players depends on a couple of factors: (1) the size of your lead; (2) how many games left in the season; and (3) whether or not you've clinched a playoff spot or not and are just playing for your seed.

My thoughts on (3): Unless you're looking at the difference between a first round bye (1/2 seed), or a 3/4/5/6 seed, it doesn't really matter unless you're looking at team matchups.  Home field advantage (3/4 seed) is not that important unless you play in an extreme stadium, so resting your guys and settling for a wild-card (5/6 seed) may not be that big a deal.

I was just curious for managing in the future. 

I rarely play my guys below 100%. I sub judiciously to keep them there.  Seems to have worked fairly well.