I understand the concept on an EMH, on ships with a small medical staff they would need back up if faced with a major crisis. But when he started wanting to be more than the ship's doctor I just didn't buy it, especially that whole "ECH" thing. Why not just turn command over to the computer, as that is all a hologram is.
In the next series they should do everything possible to distance themselves from the holographic rights crap that VOY tried to ram down our throat. A hologram, even one as sophisticated as the Doctor, is still a mass market computer programme. Data's status as a sentient being is different in that he is pretty unique, but after holograms it's be the phaser banks, then the warp reactor, then the toaster all looking to be recognised (ok a bit fanciful that, but there I see little difference between a hologram and any other ship system, its a tool).
I could understand a crew member being cyberised following the war, in order to repair serious injuries, but I think that Trek can leave androids/robots and holograms alone for the time being.
What Makes Data different is he is a totally independent Android with a Body and actual intelligence beyond mere programming. Holograms without 29th century tech don't have bodies.
I have always thought that medical staff and sick bay seemed way too small.