I'm not really sure where you got that 'Presence' is a main healing stat....because it's not. There are 3 other 'power' stats that increase healing, but Presence is not one of them. All Presence does is increases your companions attributes in some way (usually higher health/armor/damage). At least, this is from DS POV.
Companions are pretty amazing. But there are also some reiterations with companions. What someone else said about each companion falling into the 5 points is somewhat off. As a Sith Sorc, I have a tank, 3 ranged DPS/healers and 1 melee DPS. I don't have a ranged tank. While that would be nice, it really isn't that necessary.
You can gear your companions much as you can gear yourself. That's how you improve upon them and make them stronger.