By the same token, 2 Kotsura shots on a planet defended by a full fleet + defenses means 1600 damage to buildings, which isn't half bad, and 400 damage to ships (though this is mitigated by shields) which puts a serious enough dent in more brittle ships like LRMs. Disabling a bunch of repair platforms for 3 minutes definitely helps break that defense.
The thing with the Novalith is that lots of things can destroy planets. But more often than not, you have to destroy their fleet eventually, and the Novalith is useless for that. This is of course unless you can build enough Novaliths to take out all of the opponent's planets at once, which is quite expensive. By contrast, the Kotsura and the Deliverance Engine greatly help you destroy the other guy's fleet, at which point all of his planets become easy pickins, and you need much fewer of them built than you do Novaliths.
In multiplayer of course you usually don't get to the point where empires have 20+ planets and maxed out fleets, so the Novalith is a lot more attractive since destroying 2 planets out of the empire's 5-6 hurts a lot more than 2 out of 20. But when you get to a face-off with 2k vs 2k fleet point fleets, the Kotsura and Deliverance Engine give very noticeable advantages.