I look at the companions a certain way. They are your friend in the absence of a friend you can quest with. So they fill the void. The first companion you get is the opposite of what you are. If you are dps/heals you will have a tank. If you are melee dps/tank, you will get a ranged dps first. The point is, they compliment you very well throughout the game. The other companions I look at as my crafting/gathering slaves. As well as the filler role if you need for flashpoints and what not. Not that the companions are any great help in an group instance, it's better than not being able to do it because nobody is responding to ads for group.
If I was in a group as my Sorc dps, and we needed another dps, I'd call Ashara. If I needed a tank, I'd use khem. If we needed a healer, I'd have someone else in the group use their healer if they had it.
Throughout questing, there have been times where using someone other than Khem was better, but for the most part, I like having a meatshield and whether it's called Khem, Ashara or Chubakka, it's all good. Seeing as Khem is the toughest of the meatshields, I'll keep using him.