The suits probably aren't supposed to be doing their job in those environments where the helmets aren't being worn. And we do see a specially sealed environment right next to the helmet-free spaces, with its revolving-door radlock.
So the whole purpose of having the suits on in ST:TMP, ST2:TWoK and ST6:TUC might be that key personnel are standing by to enter irradiated spaces (because it's the first hot test of the new engines; it's a training opportunity for the cadets; and whatever the reason in the final example). They are not needed where worn, but taking them off would not make sense, either, because donning them again when entering the special spaces would be too slow and clumsy.
The "hot" spaces would
be shielded, then - but situations would arise where a key valve or hatch would need to be opened (just like Spock pried open that fancy pedestal) and the shielding breached, and the suits would need to be standing by for such an occasion.