Like everything - context matters. If you're just trying to condition then go ahead and condition and push your fatigue levels, but if you're trying to learn a new skill then we probably shouldn't be fatigued. When learning a new skill we want to be as fresh as possible and if you're fatigued the skill you "learn" might end up being an entirely different one all together If we're only at 40% trying to learn a new skill, when we try said skill when were at 100% we won't be able to replicate it. We were a totally different person when learned it at 40%, we need to keep that in mind. Here's 3 ways we can use fatigue as a tool: 1. Conditioning  2. Use fatigue as a constraint to challenge skills you already have  3. Performance/Strength & Conditioning - we need to perform under a fatigued state, implementing some fatigue in your resistance training in the right way can be extremely beneficial