Matter of perspective
I think this whole debate and house rule proposal is fairly silly but that has a lot to do with my perspective on the issue and the game in general.
From the original post (edited):
1: "In actuality, without the defender's "help" its damn near impossible to Sunder..."
2: "...one Sunder attack is representative of many wild swings and strikes against an opponent's weapon..."
3: "...But we know that isn't how it works. And when a warrior approaches you, holding his shield in front of him with his sword behind him and you win initiative, how the heck are you going to Sunder his blade?..."
I disagree with all of this. With #1, if the defender is using his weapon at all there will be opportunities for the sunderer to break it. A combat round does not represent a single attack, its a combined representation of many strikes and parries. The attacks that are rolled are the opportunities that arise in the midst of these strikes and parries when one opponent can hurt the other. When the sunderer is focusing his attention to damaging the item instead of the person weilding it, he will have opportunities to do so during the round in much the same way that he has opportunities to damage the opponent.
#2, I don't think a sunder attempt is representative of many strikes against an opponent's weapon, but rather represents a moment during the flurry of blows between the combatants that he has the opportunity to make the attempt.
With #3, if the defender is busy protecting his weapon he is fighting defensively and gets a bonus to his AC (which I believe affects sundering attempts, does it not?, if not then it should) while suffering penalties elsewhere. If he's fighting normally, see my view regarding #1.
The argument presented and the rule proposed is based on the idea that when a character makes an attack roll, that is the only time he is striking at the opponent. This is simply not the case as it is stated quite plainly in the combat section of the PHB not to mention things like AoO's are entirely based on that abstraction and would really have no relevance to such a point of view. At least in my opinion.
With the question regarding called shots, it can easily be assumed that when a character is attacking, he is already trying to make called shots, and when he hits where he was attempting that is represented by the critical. Since items aren't on the list of places to try and put your blade to do the most damage, it takes a special announcement to do so, hence we have the sundering rules.