Haven't leveled up my Sniper enough to have any companions besides Kaliyo (who I can't effing stand, mainly cause I take a hit to approval every time I pass up an opportunity to act like a complete Richard), but ^ this has pretty much been my experience with my other characters.
I'm sorry, but when the "tank" companions average around 25% damage reduction with on-level armor (in other words, roughly the same as my own squishy tail), there's really no good reason not to just take a slightly squishier DPS companion and kill mobs faster, or take the healer companion and just tank the mobs yourself (which you probably will with a tank companion anyway, since they suck at keeping threat as bad as they do at surviving).
So me, I'll probably be fulltiming Lokin once I get him, or bringing out Vector or Temple when I have a friend along to cover heals.