Bottom line for me is (other then khem val) if you can't see a piece of armor change the way a companion looks, then you don't want to invest in them. I hate Quinn for many reason, but I needed him from the time I got him until the last chapter. Smuggler
But heal comps are vital depending which advanced class you roll, that you tend use them exclusively.
The One's I never use are:
M14X (pulls targets to you,again I hate that)
Lord Scourge (useless unless you Role-play Dark Jedi
HK (keeps jumping back to my spot I hate that)
Risha I hate her from the time I got her, just another sniper DPS same as the devorian and the twilek guy.
Most all of the tank/DPS companions I never used at all but Qyzen and khem Val, Myzen bomber for the win.
Quinn, Dorn,Tuno, khem, Theran, Vector, Qyzen, Mako I use all the time.