The Doctor on Voyager is a hologram. He is just as competent as the human doctors on other starships. In fact, I would argue that the hologram Doctor is even more competent than the humans, as he was able to devise a weapon that defeated Species 8472. Even the Borg which crushed Starfleet were defeated by Species 8472. The victor's victor was no match for a hologram.
Starfleet missions are very dangerous. They are exposed to risk all of the time. Given that human lives are more valuable than holograms, wouldn't it be a great idea to replace most of the human crew with holograms which are "cheaper" and arguably more competent? In today's context, the Pentagon is exploring the idea of using machines to do the risky job of soldiers on dangerous missions.
Why doesn't Starfleet do this on a massive scale, given that it makes so much sense to do so? I am also surprised that the Vulcans have not been doing this, given the logic. The hologram technology should also be available to Vulcans, as Vulcans are more advanced than humans. The logic is clear. Resistance is futile.