Condition has to effect the player performance. When you are logging on to the game sometimes reads "did you know that in football the condition of a player is as important as morale of a player". Or something on those lines.

As a result of that I try to keep condition of the players at gametime at a minimum of 80 if I cannot hit the max condition. Less then 80 condition and they are benched.

I also train my reserves until they hit 25/30 condition (barring injuries to the main squad)