Well, the armor was initially used to defend Admiral Janeway's shuttle against the ship that tried to stop her from going back in time. But Seven does say, "The technology aboard the Admiral's ship is impressive. Much of it appears to have been designed to defend against the Borg." It's never made completely explicit that the armor was specifically a Borg defense, but that does seem to be the implication. Even so, there's no reason to treat it as a default assumption that the armor is some kind of ultimate defense. And there are plenty of reasons not to want the armor to be used again.