I think that kaiser dragoons are absolutely worth using in a sub slot at 0*. I also think that's the consensus, given how often they're emphasized to be the most important thing to grab in the eidolon orb exchange. And there's only 1 copy of each in the exchange, so if you never draw one from the gacha, then at best you'll have a 0* kaiser.
The +30 to elemental attack is pretty nice. But I personally look more at the other aspect of their summon effect.
If you can't take out the enemy before it can wipe you out with whatever its major attack is, 2 turns of 30% element resist is really good on its own. That's usually sufficient to save your butt on its own, but not always. Then it becomes absurdly good when you take advantage of stacking rules.