50 Jedi Guardian
Used Kira as soon as I got her until I got Doc, then pretty much Doc from then on. Occasionally ill bust out Rusk, Scourge, or Kira again on a whim when I'm doing easy dailies, but I pretty much use Doc for everything that mattered. Having him around reduces MY downtime, which means my already slow killing is sped up in that I don't have to rest between fights like I normally do with any of the others.
I think their best bet would be to turn ALL of our companions into some form of healer-hybrid. Kira's kinda built on a consular base, have her attacks have a chance to heal her and you. Rusk uses assault cannons, give him some commando-ish medic stuff. Scourge has his dark side drain...Keep Doc as the most dedicated of the healers, but give the other companions something. It'll smooth out low level difficulties and provide options at high levels. I really feel like everyone but Kira early on and Doc ends up just not getting enough face time with me. They should all be useful enough that you don't mind taking them out with you on occasion. Everyone will have their favorite obviously, but as it stands, T7 I didn't use beyond getting Kira, Kira I rarely used beyond getting Doc, and I never used Rusk or Scourge at all. I mostly feel sorry for Rusk, he's built up as this crazy soldier who just gets his squad hurt, and if I didn't decide to get his affection up after hitting 50 id never have any inkling of who he is beyond that...he's actually kinda interesting once you get to know him, but he's just not who you're gonna be bringing along with you once you get Doc, so you probably wont find out without showering him with military gear.