TOS always had really ugly spacesuits IMO. But you got the idea of what they were supposed to do.
I don't know what the purpose of the environmental suit in The Naked Time though. Obviously it wasn't secure. But why would they suddenly put a 'protective suit' on in this one episode? Other times they beamed into potentially hazardous situations and didn't wear a suit - "Omega Glory", "Miri","Doomsday Machine", "Wink of an Eye", "Operation Anihilate"