Today I have 4 pork butts cooking in my KK42, and yes I'm using the upper grate. It isn't just the bark. I get better smoke on the upper grate with low and slow. And with spatchcock chickens I get crispier skin on the upper grate (dome really radiates the heat and the closer the food is, the better, in my experience.
But I will say coming from the BGE it doesn't matter which grate main or upper, the results are better. I can get close with the egg but the KK is the best. Sorry for the slow response - haven't been on here in a few months. Cheers!