Very true that a merged profile with cabs off through a guitar cab should be identical to the direct amp profile with the same settings. My experience with my own merged profiles compared to the same settings' studio profiles is that they are not truly identical. The introduction of the DI into the equation still alters the tone slightly. The merging process, while way better at cab separation than before, still doesn't capture the sound of the studio profiles perfectly, at least to my ears. That is not to say that merged profiles sound bad at all. They don't, and for those who prefer to hear the "amp in room" sound with the cabs off, this is a very viable alternative to studio profiles through FRFR cabs. It is just that in my application, I don't have the need to hear the DI into cab sound in that manner. I am usually in situations where the studio profile works best because I'm either running into a p.a., using iem's for monitoring or recording, where I want the full studio profile anyway.
There are so many ways to monitor the KPA now, it's just a matter of tailoring the profiles you want/need to your monitoring device and I hear feedback from people saying that many different systems work great. It just depends on their needs and what they think sounds good. The style of music also has a bit to do with it. Heavier gain sounds benefit more from having the clarity of high end that comes from using DI/merged profiles with the cab sims off. For the country or slight overdrive stuff, the cab sims kinda round out the top end so it sounds a little smoother, but it's all subjective.