It's because your last post was made AFTER mine.
"Movement oriented effects, like bonuses to Dex, benefit the image. " Is compelling and makes sense. But I think you put too much weight on the visual description of the spell rather than the actual mechanics of how magic works in D&D (the game, not the place).
and...
You claim that MI is a movement effect, yet if the caster was hasted, the AC of the figments does not increase. "The transmuted creatures move and act more quickly than normal. This extra speed has several effects. ...A hasted creature gains a +1 bonus on attack rolls and a +1 dodge bonus to AC and Reflex saves."
Since it is not a size or dex modifier, the figments do not gain the benefit, but haste clearly says it is a modifier granted due to increased movement.
This is the difference between an ACTUAL movement effect and a fluff movement effect.