First of all... It's not a rule about fatiuge, it's a request. There is no penalty.
What I want to avoid are the owners who draft 450 PA guys and put $200k scrubs out there in 30 games, choosing to lose 80% of those games so that they can win 80% of the other games. I want to avoid the 1000 good IP and 5 $200k pitchers to suck up losses.
If you want to use a tired mop up to save the rest of your staff, that's fine. If you are forced to put a starter on 40 PC and go to a mop up once in a while, that's OK, I guess. Injuries are on, you can't replace the mop up, you'll have to deal with him getting injured and one of your other pitchers going into the game.
The key point: You're being asked not to bring a fatigue oriented strategy to this league. Each of us knows what that means, and it's on our own conscience if we don't work with it.