Look, the Inquisition codex actually has a lot of fluff justification. You should be able to have basically any imperial army run by some inquisitors.
There's cool guy, Andre, he has an army that is basically only Inquisition and stromtroopers. That's what he thinks his cool. Didn't like Coteaz, which was basically the only way to play that army, so he was pretty ticked when the GK codex came out. Inquisition codex let's him play that.
I have both Coteaz and Karazamov. I also have a lot of marines, converted to look like GK, cuz guess what, I really do just like knights. I'm pretty sure I can make a fairly fluffy, cool looking list with a combination of GK, "Grey Knight" marine bikers, and Karazmov dropping orbital bombardments on the Chpater master with the shield that just don't care.
That's all good stuff. But the codex is still a copy& paste hack job, and letting everyone take COteaz just because they felt like it isn't a great idea.