Since the cVar unitHighlights was only later added in patch 2.4.0, I can't find any other way to disable unit highlights.
"Visual Settings
* There's a new cVar unitHighlights; 0 = no model highlighting with Alt-Z, 1 = model highlighting on with Alt-Z. (e.g. /console unitHighlights 1"
http://wowwiki.wikia.com/Patch_2.4.0/API_changes
So adding this would be pretty cool, at least for me because I find this unit highlighting very annoying...
If you have any idea how to disable it otherwise, please let me know
Thanks