Should know I never promote cavaliers to great knights in FE8 because of mentioned reason and the fact that they're vulnerable against lots of effective weapons.
I'm pretty sure Mastergabe2000 just wanted to point out that Rutger isn't available against all the bosses or is the best unit against them (however he also forgot to read the word "most" in my comment). The first two mentioned bosses appear before Rutger joins and Idoun is laughable easy to beat with Roy's Sealed Sword.
I've not said all. ^^
The only bosses he didn't kill since he was playable were Narshen, Alucard, Roartz and Brenya. Everything else Rutger killed (thanks Clarine-support).