I finished this questline playing with the Imperials. In the endi ve been told that the civil war hasnt finished and that i should attack the reamining stormcloack camps to destroy stormcloacks competely. However after attacking one of these camps the local stormcloak leader refused to die since he is marked as essential. So it seems that the stormcloaks can not be fully defeated which ia a bit annoying. Does a reward exists for clearing the remaining camps?

Nope, don't even bother, they're all immortal.

My personal opinion is that the camp leaders are all marked as essential for future DLC. I still believe we will get a DLC expanding the Civil War questline as well as have a moot take place.

Nope, it is one of those unrewarded actions. Unless you count the low value equipment they drop as a reward of course. Ad for the essential commanders, it is likely an oversight. Still very annoying though.

On the bright side. You can max all of your offensive skills on those essential characters if you're willing to use that particular exploit.

Don't they have a little low health and long resting period for that to be effective?