I -think- the original idea way back in the beta days was for mercenaries to be further developed but the idea was nixed in favor of more developed companions. If I remember correctly, mercenaries predate Companions?
If development were still going Mercs need changing. At the moment they're not useless they're worse than useless: they're useless with a GP cost.
It might not take much to improve them. Perhaps very small inventory list where you can drop (same as lose, no returning) weapons, shields, armor and health potions. That way you could upgrade them and kinda hopefully keep them alive as they drink health potions to keep going.
At the moment they're too squishy and mortal. But, again, we're fantasising over things that are NOT going to happen.