Personally, it was my understanding that at the time of TMP, the Enterprise was equipped with all the latest gadgets, a sort of testbed, so instead of just forcefield-based shields, they were also equipped with a new, to borrow a phrase from TWOK, "Defense fields" based on the same technology used for deflectors, deflector screens. Witness Sulu's comment after V'Ger's weapon didn't destroy the Enterprise, "The new screens held!"
I figured they were developed as a defense more effective against physical projectiles, since they were based on the same technology as navigational deflectors. Balance of Terror showed that the plasma torpedoes were significantly more effective against the Enterprise's shields. Lo and behold, by the time the Refit is rolled out, torpedo launchers are significantly more visually prominent and seem to be used more often than in TOS, when it seemed beam weapons like phasers and disruptors were more commonly used.
By the time of TNG, they probably just put both up at the same time, maybe even combining the projector units. Just my two cents.