Mercenaries are in an awkward spot. During leveling phase they will often outshine the player massively. Once the player hits uber quests they quickly die and more or less stop mattering at all.
What if mercenaries were made unkillable & untargetabble, but nerf their damage output to 10-15% of what they currently do? you could pick one that compliments your build with its own skills and not worry about it being a dead weight in uber levels.