It's a fatigue and pricing issue...
Hitters fatigue by PA. Pitchers fatigue by pitch thrown. If you have a hitter who starts 5 games in RF, he'll fatigue by his PA for those 5 games. Then if you start him as a pitcher in game 6, he'll fatigue by pitch count for that game. So, as a hitter, he is getting (per the sim fatigue structure) one game off out of every 6. And as a pitcher, he is getting 5 games off for every 6.
Basically, that means you can get away with (in this example) getting the use of a 600 PA hitter for the price of a 500 PA hitter, plus getting a guy who can start 30 games for free.
Just a nightmare situation.
Of course, pricing is also a problem with no satisfactory resolution, but the big bug is in the way hitters and pitchers fatigue differently.