Some of the US matches DO throw the classic target into their stages because they are more challenging.
As for an exemption to use the classic target, why do we need one? You want to simulate an upper panel only shot? Take a regular classic target, paint it black and put a 2/3 target just slightly above it. Or better yet, put a classic target painted black in place, and put an IPSC round plate above it.
Personally, the metric target allows people to compensate for poor accuracy. The metric target has a bigger A zone and larger scoring area. As well, I can't think of the number of times I've seen someone aiming at the lower A zone, and drift rounds into the upper panel.
Am I sad to see the Metric Target go? Sure, but it's just a target. We could be shooting at tombstone targets and it wouldn't change how we approach the game. Does it take away some options for stage design? Sure, but to be honest, as a match director, I've never used the metric target in a match (except when there was no classic target).
If someone could give some GOOD reasons why the metric target NEEDED to stay, then I'd be happy to hear them, but I would wager that most of the reasons are really not that compelling.