from a purely mechanical perspective, (and yes I know that the game works differently) Mirror Image should not do a thing for traps. The trap isn't (in most cases) sentient and can not be tricked. If you step on a pit trap, you fall, not your illusions. If your illusions step on the pit trap, they float in mid air. And it isn't the illusion that pushes the button (or otherwise trips the trap). Sorry, just a pet peeve of mine.
But yeah, I understand that Mirror Image works well.
I've always viewed mirror image as a sort of self-tank/proxy damage absorption spell. Even though you step on the pressure plate, your illusionary makeshift meatshield soaks up the damage. I'm almost positive this runs contrary to the lore, but that's how I personally justify it.
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But yeah, I understand that Mirror Image works well.