Any melee attacker needs expertise up to a point to prevent being dodged, even when attacking from behind. When attacking from behind you cannot be parried, but you can still be dodged.
Read the expertise section of this post for some more details:
edit: This is the post describing the custom stat threshold features, but you usually do not need to use it, the optimizer will handle it automatically. But it explains all of the stat thresholds relevant to classic, TBC, and WotLK.