...One wonders what this suit really protects against, though.
I mean, it apparently did zip against the one type of radiation we saw threaten the ship, in ST2. It was used in ST6 purely for camouflage, and no functionality beyond that (say, the ability to withstand vacuum) could be discerned.
It wasn't a "precaution" type of protective suit, worn in special times
, as there were no helmets readily available in ST2 - the crew grabbed simple breathing gear instead. But it could have been a type of suit to be worn in certain rooms
, the anterooms of which would feature helmet racks, and Scotty et al. didn't bother to take it off when working elsewhere.
Whatever the in-universe use of the suit, the one thing it was never associated with was radiation protection...