According to "Court Martial", his specialty is space psychology - a very useful skill to have on a five-year mission, and probably it makes him a good choice for Starfleet to replace the Piper/Dehner combo. Perhaps such concerns outweighed the need to care for Spock's well-being?
Timo Saloniemi
True, and humans in TOS seemed to think of Vulcans as almost invincible as well so they may have thought a Vulcan wouldn't need that much medical attention.