After reading the first couple pages, I feel I must explain further. Both Qyzen and Iresso can either DPS or Tank. If you've looked at their abilities, they have a tanking 'Guard-like' ability, and a 25% Damage increase ability.... They can usefully fulfill either role. I know for a fact that Qyzen can melee dps almost as well as Nadia. Also, I am on an RP server for a reason, and even though I'm 50, I still like having companions out. That being said, I have solo'ed multiple Champion bosses on Voss with her, though I haven't tried any on Correllia yet. It's not that she's a bad companion for a healer, but she will never be as good as Qyzen as it presently stands, and anyone who says that she is doesn't know what they're doing... I just wonder why Qyzen and Iresso both have combat stance... which turns them into DPS companions and lowers their threat, while Nadia has no 'Guard-like' ability to help her hold threat and take a little less damage....
Also, my alt is a Shadow, tank spec, and I sympathize with all of you saying that most of the companions are useless. Theran is really the only viable option... There should be a better selection of healer companions....... maybe even a heal stance for nadia?! :-) All of the companions being able to fulfill all trinity options would be an excellent idea, I actually think it would lead to more variation, not less.