As far as I know, epic or legendary orcs provide one thing, epic or legendary gear when killed. Otherwise, they're just like any other orc, potentially with some better stats.
But what's the point of keeping them alive? When it comes to conquests, it doesn't seem to make it any easier by having epics or legendaries help, they are just as squishy as other regular orcs. And it doesn't seem like the regions are any more safer when they are a warchief or overlord. With all these war chests giving me these orcs, I'm seeing no reason for keeping them alive.
Why shouldn't I just kill them for their gear?