I agree with all of that
We have a party with some characters optimized mechanically and some optimized non-mechanically, each to their creator's satisfaction. Then we put them together, each played in their own way by their advocates, and see what happens.
What happens is that, not only do the characters not co-operate well when the chips are down, but the players don't either. Without necessarily meaning to, we've tested the meta-game to destruction.
We've demonstrated the importance of choosing players to play with who think on similar lines to you. We knew that anyway, but here we have a case study of things going wrong if you don't.
So we have achieved something.
Well I agree with this 100%. The players definitely need to be on the same page or there is just chaos. A good DM can make that work, but for this test it defeats the purpose.