It's like hand guns. Everybody buys hand guns for home defense, yet somehow some of those defensive weapons manage to be used in a manner contrary to the user's stated purpose when purchasing them.
Your analogy is faulty, Dax. Handguns are ALWAYS offensive weapons because they are specifically purpose designed to project power outwardly. That they can be described as "defensive" falls in the same category as ICBMs were used in the strategy of M.A.D.D. Neither the United States nor the Soviet Union wanted to engage each other with their offesive weapons, therefore a defacto "defense" was achieved.
However, modern missiles systems are very purpose specific. Just as you wouldn't try to use an ICBM in a short-range attack or a short ranged missile to strike a target on the other side of the world, so you cannot adapt them for a radically different purpose.
Rather than with a hand gun, a missile defense shield would more accurately be equated with a bullet proof vest. You cannot use a Kevlar vest offensively unless, I suppose, you wanted to try to hit someone over the head with it. Not likely, though.