There was another "attack rearmost" thread a while back ago, in case anyone wants to add to that:
http://forum.shrapnelgames.com/showthread.php?t=41915
My experience, including controlled tests, is that individual squad members get distracted when attacking rearmost, as llama indicates. But, what you say about attacking rearmost commanders is nearly correct, omni. If used properly, fliers can actually get to their targets, but ground flankers are more likely to get distracted. Relevant factors seem to include number of attacking squads with "attack rear" orders, number of defending squads, and defending squads with "guard commander" orders.