As a person who plays barbarian and gets bruised a lot, and hates to wait for pug healer to show up, I very very often run with hirelings. they are great, if you can control them. Unfortunately they seem to be unable to perform tasks I've given to them. There are few reasons. Here's couple of ways on how to fix them.
Have hireling movement toggle button to cycle trough multiple choices:
1. Follow very closely - setting useful for guiding hire onto pressure plates, safe spots, and basically, for parking them.
2. Loose formation follow - hireling will follow you, but it will try to space out to avoid getting hit by AoE attacks/spells.
3. Semi-Parking mode - hireling jumps out of place where he was parked only to do heals or to perform tasks ordered by his master, and afterwards, returns immediately exactly to his parking place - if he can't get there by foot in 3 seconds, teleports. Semi parking + defensive mode = stealth + heals only, none of those crappy sunbeams that do next to no damage and still manage to get aggro.
4. Full stop mode - hireling stays in place; can turn around to face enemies in attack mode, or friends in defensive mode, but stays at exact same location it been told to. Shoots in offensive, heals in deffensive. Useful for keeping pressure plate pressed, preventing from messing up the puzzles, prevents hire from running into traps to try to heal (and from failing to heal due to concentration check on trap damage, and from trying to heal itself while still in middle of trap and from failing this too and from dying in trap).
Give higher level hirelings more HP and some more metamagics for spells (seriously, how did they get to lvl 18 with this low concentration, and without quickening heals, did they stay in forest killing boars or what?).