Of course I carry enough air for my buddy! & yes, we follow the rule of 3rds & dive our plan. Whoever first uses 1/3 of their starting supply & we turn back regardless. I do agree with you about the pitfalls of H&W, B&G buddies, so that is why planning is important. In a perfect world, we would be buddied-up with those who have the same dive-style, SAC rates, air supply & gear. [I think we're starting to get into DIR territory here....]
& for an overhead environment, I would not buddy with a known hog, or potential hog (aka he/she-who-becomes-a-hog-when-conditions-are-less-than-ideal). In my limited experience, I've seen such individuals not follow basic rules/common sense much less the rule of 3rds b/c they are so focused on trying to entend their time/depth/range with little thought of the consequences. These are the same people that routinely ask for air at the bottom of the line & surface at barely 500psi. These are the people I'm kvetching about.
& as for diving in an overhead environment with a 60cuft remaining air supply & a buddy with 120cuft - now what is the point in that?