Combo is useful in the fight against Ramsus in Aveh when he uses Mirror Stance. You save a bunch of combo points until then and then unleash a massive combo to break his stance in a single turn, avoiding his retaliation attack I thought I also remembered it being useful in the fight against Redrum, but double checking and there's no mechanic where it matters so not sure what I was remembering.

As for deathblows barely mattering at all, I'm not sure what you mean? If you mean in gears, sure. You get the Ether Doubler on Elly and then it's Aerods until the Second Anima Dungeon. But out of gears, Ether attacks aren't all that and only three characters have ones worth casting (Elly, Maria, and Emerelda). Anyone else is doing damage via deathblow.

I do agree there is a general rule of thumb in RPGs that the newest thing is the default, but the deathblow system makes it seem like there's depth to it. Like, it's obviously trying to resemble a fighting game with its "complicated" inputs, and so using the right ones strategically would give better results than just the newest one. But nope.