I'm in the "can't see myself ever needing a different amp" category. I play a really wide range of styles, so I'd need a collection of amps to get my current sounds.
There's still a bit of something extra with a real tube amp to my ears, but I have to be right next to both to hear it and even then it's too subtle for me to care about given the benefits of having everything I need in the same unit.
It seems like a lot of the people that don't like the Kemper play metal as far as I can tell, so maybe it doesn't do Mesa brutal as well as the types of amps I love. If I was a gigging metal player I'd just get the tube amp that has the right high gain sound for me.