I may be mistaken, but weren't the lines in the hull plating (grid lines) referred to as part of the shield grid or defensive grid? If this is the case, could we assume, as Timo suggested, that this system provides the skin tight shield system? The screens, as Kirk often would say during TOS, might be the invisible energy bubble that extended well past the hull. Perhaps there was no mention of seperate systems, after TMP, because it was an unmentioned upgrade after the encounter with Vger. Also, when Saavik mentions about activating defenses, this may be referring to something involving the hull plating.