a space suit can't just be turned "off"
...Which makes it really inconvenient to use in boarding party duty. Perhaps the very reason our heroes chose the belts for their "BtFS" foray to the alien vessel?
OTOH, if life support belts were standard issue for boarding parties, we would have missed all the excitement in "The Naked Time". Instead of donning thermal garments that allowed contamination, our heroes would have worn airtight forcefields on Psi 2000. And the plot complication in "Omega Glory" would also have been removed, as the interior of the Exeter
would have stood no chance of infecting them with the pulverizing disease.
While from the practical viewpoint there exist parallel niches for nifty but not so protective life support belts and clumsy but safe physical suits, Star Trek would probably become much less exciting if the belts really were utilized outside TAS. Although I'm quite convinced we saw them in ST:TMP still, as evidenced by those oversized belt buckles...