Hmmm... So only bowlers using their thumb are allowed remove two large cores radically affecting the ball's differential? (a.k.a.MOtion Hole drilling, Double Thumb, P3 gradient holes etc..?)
I'm probably going to surprise some folks by standing up for the 2 handers here, but this rule change smacks of "You can do something we can't, so we're changing the rules to disadvantage you."
i.e. Bowling will happily assist people with inadequate releases. (Wristguards, blocked lanes, ultra reactive covers (which destroy lanes, btw, especially on the RHS), Ultra low RG / high diff ball cores, increased leverage layouts, sandpaper to the horizon. But as soon as you think outside the square, bowling wants to get in your way.
I'm not saying the rule change is wrong. But I can't put my hand on my heart and say it's fair. These days "balance holes" are mostly used for creating leverage for people who can't (or won't make the effort to) create their own. Most 2 handers don't need or benefit from a balance hole, unless the ball has a MB on the wrong side of the pin/CG line and needs some weight out to meets tolerances. They generate enough leverage to get the ball to do something by themselves. One of the "leverage craters" I dig for the hand-challenged members of our community just doesn't work for the athletically more capable, as these layouts just turn and die for them. So this rule doesn't achieve much except to placate the mob with their pitchforks and torches shouting "he's cheating!" When in fact, he's probably cheating less than they are.
The trouble with being so far down the slippery slope of mechanical aids, free hook, free hit and free area on the lanes is that the most conservative of us are probably in the worst position to cast the first stone. If we're going to wind back perceived advantages, then do it for everyone, not just the guys who work really hard do something most can't be bothered attempting.
I suspect that substantially reducing the maximum weight hole diameter (to say 13/16", ottomh) for everyone would be a more equitable solution and still allow pro shop operators to work with most balls that turn up on their door. (But that's another topic.)
Anyway, back to packing up my house...