There certainly is a learning curve regarding their usage, and together with the general idea that persists among the player base that hirelings are useless in any context, any difficulty they have with them is usually enough to confirm their biases and prevent them from further trying to improve their skill at utilizing them. They certainly have a limited AI, and we sure should not expect them to behave like an experienced player would, but I am constantly witness to them not being as bad as you claim.
The Muse uses irresistible dances on reapers and champions all the time, providing timely CC for people who don't have any (or can't get their DCs high enough to be fully reliable). As I said before, Albereth is pretty good at healing both me and the other hirelings and even other players, so even a regular one can be useful. The Shield Guardian is the best tank among the perma ones, and the raptor is the best at actually hitting things.
The legendary summons, well utilized, can be farmed for free and can keep aggro and tank bosses for long enough for you to kill them.
I certainly would never be able to complete quests on such high difficulties if they were as brain dead as you say. If you don't solo much or are always on full pugs, then sure, you don't have much reason to invest on learning how to use them better. But for people who solo frequently or play on small parties it's a good skill to have even now, depending on their build, and could be even better with the changes I propose.
Cheers,
NH