The reason that everyone uses mirror image wrong is simple playability. If you don't use mirror image that way, you have to deal with a number of additional issues:
-Do images provoke attacks of opportunity for casting spells, picking up objects, or moving through each other as the spell describes them doing?
-Do images appear to take attacks of opportunity or can one sort out the real wizard from the images by observing which of them is able to take AoOs?
-Can one move through a square with a mirror image in it? If I can't, the spell becomes a great means of battlefield control; if I can do so without overrun, does that pop the images?
-If the images are in a line A-B-C-D-E-F-G-H, can one pick off images C, E, and G, thus ensuring that there cannot possibly be more than one image within 5' of either the caster or another image and therefore eliminating all of the images (except A/B if the real mage is one of them)?
-What if one or several images are in the radius of an area effect spell but not all of them and the wizard? Do all of the images appear to take damage as described in the spell? If so, does that mean that the wizard takes damage as well? Or can you tell the wizard because he's the only one who isn't smoking? If not, do you pop all the images in the area of effect? If one of them is actually the wizard do you then not pop any (though you can narrow down the wizard's location to one of the "images" that was in the area of the spell)?
All told, it opens a huge can of worms and (probably) makes Combat Reflexes the anti-mirror image feat instead of Great Cleave. Much simpler to just assume that all the images are in the same square and then all of those issues go away. If they have the opportunity to take or provoke an AoO, the wizard does too. They're all within 5' of the wizard at all times. And, if any of them (or the wizard) are in an area effect spell, they all are.
Patlin said:
One of the key things I've picked up from some of the prior debates is that this problem is in part due to the fact that everyone uses mirror image wrong... at least I do. I always assume for simplicity that all the mirror images are in the casters square. Technically, they only have to be within 5' of another image. Therefore, the great cleave attacker may not be within reach of all of them.
For simplicity, which is important, I'm willing to house rule that they are all in the same square and that great cleave doesn't work on images.