Well first of all we are talking about doing techniques completely different from what the teacher of the moment is asking the class to do. With out a doubt there must be an adjustment based on both tori's and uke's body type but that should be within the framework of what was asked.
Consider this scenario, which happens in nearly every class:
1. Teacher's uke attacks during demonstration
2. Teacher responds appropriately to attack
3. Your uke attacks differently
Will you tell your attacker they are doing it wrong? This is too selfish and wasteful.
Will you pretend your uke is the teacher's uke? This is fantasy, not martial art.
Will you respond to your attacker as the situation demands? If you can accompish this and nothing else, you've nothing to worry about.
Rule #1: "Get out of the way." Not "Copy the teacher."
Well I know where you are trying to go with this (see above) but freestyle is where at least attack or defence is unpredicatable.
Unless your aikido school trains time travel and psychic powers, you presume too much.
Is your teacher flawless? Or maybe you are not very observant? Perhaps you are unable to learn from others' mistakes.
Well considering my level of experience compared to my teacher - the answer is yes.
Fair enough. But not all of us are in the same boat, and arrogance has nothing to do with it.
Anyhow, I fail to see what your above quote has to do with some yahoo deliberately doing his own thing against the wishes of the person teaching the class.
Nobody gets it perfectly right. Whether they fail by accident, or on purpose, is none of my business.