Here's a question, strictly in terms of hypertrophy.
Take any exercise, we'll say bench press for this example. We put 50kg on the bar, nice slow and controlled eccentrics, perfect form, say we get 90% chest activation doing this. We then put on 60kg, on this we only get 60% chest. 20 shoulders and 20 triceps, we add another 10kg so 70kg on the bar, this time we're only getting 40% chest, 30% shoulders and 30% triceps. These are just examples but you get where I am going with this.
So if hypertrophy is our goal, and we're trying to isolate a muscle where do we stop? In the example of the bench press sure we can lift 70kg but we are pulling in other muscles and our form just might not be 100% perfect. Its important to differentiate between hypertrophy and strength because with strength it doesn't really matter as much