If thugs are not undead or constructs reinvigoration is critical.
When unit gains fatigue it becomes vulnerable to critical hits. More the fatigue - more often critical hits are and more damage they do.
In my experience fatigue below 20 is pretty safe and more then 60 - suicidal. So if you don't want your thugs to be killed in long battle - you'd better keep it low.
Edit: Ninja'd by Fantomen, type too slow.