So you only get 3 uses of Karstaag, so where do you guys plan to use it?

I'll probably summon him against the ebony warrior on my 4th or 5th try, if I don't beat him by then.

The final attack in the civil war questline would be fun, just have karstaag destroy the guards of a city for a minute and a half.

And I'm still not sure about the third, I might just save it incase I use one on accident.

Alduin with Deadly Dragons Loremonger.

Maybe Civil War, but I'll probably save it for something in the next DLC, possibly that hinted Thalmor one?

Ebony warrior, I will summon him, bring my favourite companion, a riekling armed with dragonbone arrows and a horse (shadowmere died). Then use berserker rage, dragon aspect, the shout you learn in the DB base and a bunch of potions.

Other than that there is no need to use him as he was the only enemy strong enough to have that ability be effective

I think the next DLC is putting more focus on the East Empire Company, They would be able to perfectly give you that Dutch East Indian Company-feel. And in dragonborn we had to collect 10 objects related to morrowind, and now we get to go to morrowind. Now we have to obtain collectibles related to the EEC, so here's hoping we're invited on the first EEC expedition to Akavir.

(altho sadly we probably won't see akavir until the first/second expansion of the TES MMO

Save him for the next DLC.