Interesting that you mention Kremlin. It's a great wonder, absolutely no doubt about it. However, I find that in that period I rarely run slavery (unless in a sushi game, where cold whipping execs and settlers is great!), so it's kind of a waste if I capture it, and I don't often build it myself any more. It's not cheap, and if I don't run slavery any more, it's just a waste of hammers. Would be different with spiritual civs, though. Easy to make a bout of whipping factories for example.