The problem I see with turning fatigue off is that it affects your armies too. So you have zero net change and all that is really accomplished is you make battles more bloody without making them more tactically challenging.

On top of that, if you play as the Romani - who somewhat rely on attacking in depth largely with exploiting fatigue in mind, you pretty much remove that entire aspect from tactical considerations.

But I guess on the other hand it does reduce the huge AI crutch that is defending siege battles when they run around like idiots. But then 90% of the time this seems to be caused by humans using ranged attacks against enemies behind walls. I usually "fix" this deficiency by not allowing my ranged units to attack anything behind a wall (even a pallisade) as its not terribly realistic for skirmishers/slingers/archers who are ground level to shoot at something nobody in your army can technically see.