From what I can tell the Legendaries only hold their name to two aspects: 1) Unique Appearance 2) Unique affixes (i.e. a wrist piece with "+ % ATK Speed" & + "% MOV Speed")
What evidence do you have that rares can't roll those affixes? A truly unique affix would be one seen ONLY on legendary gear (like immunity to slows, or chance on hit to create a 10 yd explosion for x% wep dmg).
Sorry I thought it would be pretty obvious, but I guess I'm wrong.
It seems that you may not use auction house a lot(which is a good thing). If you filter "Armor" - > "Wrist" , the sub filters are updated, and there is no "movement speed" for example in there for example compared to If you filter "Armor" - > "Boots".
Thanks for your suggestion about the Legendary unique affixes, and I'm updating it right now.