Is it magic or computer science we're debating ?
If it is computer science, then if the caster can't target mirror images with magic missile, he'll know at the time of casting what his potential targets are (I guess a big red cross appears around them, à la HUD, and a static-parasited voice declare: "acquired, let's look the result!" once selection is made). Then, that means that magic missile defeats mirror images not because it destroy them, but because he may bypass them and send all missiles at the proper target.
Another solution based on that same premise is that you may target whatever you want, but magic missile works only on creatures. In other words, at contacting the mirror images, the spell has no effect. To an onlookers, that looks like a wizard being hit by the spell, and soaking it without even a grunt of pain. Just like smite evil on a neutral or good person don't works, although you may try it.
I favor the latter solution.
Finally, it we're talking about magic, then please instantly stop that rule-lawyerism! Don't you know you're killing fey with such a tern behavior ? I want you to clap your hands! Maybe that'll undo the damage done.