But body armor and helmets would be very useful for protection against shrapnel and secondary hits. It could also help in close-quarters combat.
But I kind of liked the notion that Starfleet didn't have a sophisticated ground military. It fits with the theme of peaceful exploration by ships crewed by scientists, engineers, and at most, security/police guys. Sure, they adequately defended their ships, but they didn't have true *soldiers*.
And my favorite uniforms are from the TNG-era shows — except the early TNG spandex!