I never really swapped gear during a strike or nightfall myself unless you suddenly hit a wave of enemies with shields of a certain type and want to make your life easier.

It's another way to make things artificially a bit harder or at least take longer to keep us "engaged" or something.

Yeah I swap gear a lot during strikes so it hits me more but a lot of times I am messing around with some new dungeon loadout and I forget to swap back to the champions weapons and then I'm useless because I can't simply swap a sidearm for a scout....