I've always thought the life-support belts were a very sci-fi idea that I couldn't reconcile with scientific credibility. Maybe I'm wrong, but as a concept it bugs me. Sure I understand the basic idea such as using a forcefield (such as a plasma window) to keep air out (or in) but that air has to be recycled. Where's the air supply? Also I guess life support belts are easier to draw than spacesuits and easier for the viewer to recognize the characters.
As I understand it, though, the life-support belt idea was initially conceived during the production of TOS, but they decided to go with environmental suits as seen in "The Tholian Web."