When exercising we all want to be pushed. Indeed fatigue is a good thing. Fatigue effectively shows that we are working at our physical limits for a given attribute. However, the issue I have with higher repetition work is that often form is compromised. When form is compromised the body places undue stress on its joint structures. For example, press ups- the lower back often drops under fatigue putting strain through the lumbar spine as well as flared elbows putting excessive strain on the AC joint at the shoulder.
It happens on lower body exercises as well- excessive flexion of the lumbar spine when performing exercises such as Burpees as well as more fundamental exercises such as squats and lunges for high repetition.
Now some people may say this is reflective of “fitness” (which in itself is a highly general term and can mean a lot of things). Ultimately though fatigue will mask function if too extreme- indeed fitness for a given parameter is the ability to resist fatigue compared to it being the factor that ultimately ends the given exercise.
Training emphasis should always be on correct form ahead of fatigue. Endless repetitions of poor fatigue can cause poor movement patterns and ultimately injury- don’t compromise what you do to just feel “the burn.” Training is about making yourself better be it strength or cardiovascular- endless repetition in an incorrect way does not emphasise good training. Remember you are looking for the best possible result with the least possible effort- once you have achieved what you need to in a session then excessive work will generally fatigue you unnecessarily.