I have pondered the same question myself. When i first started useing maya i was excited to learn that i could use programs like deep paint to be able to paint directly in 3d. But i found i could never really get the accuracy i wanted with the tools. Maybe i could have if i spent more time learning the program. Anyhow, i went back to makeing uv snapshots and just directly painting on them. This way i can get the accuracy and the detail i need.