I sent the question to Standard Motor Products, and got this reply (they reference Mopar ignition because I asked about replacing it with an HEI module and SMP's UC12X coil):

"Electronic ignition systems, whether factory Mopar or aftermarket, don't utilize breaker points, thereby not actually needing a ballast resistor to control amperage through the pickup coil. And while ignition coil life may be compromised without a resistor, the additional secondary voltage (at the spark plug) is increased which generally improves power. So does your application require a ballast resistor? Quite simply, if your distributor has breaker points the answer is yes; if not, the answer is no."

Assuming this is correct, it sounds like Mopar used a ballast resistor with electronic ignition to maximize coil life even though it maybe wasn't necessary, while providing a slightly weaker spark. And for coils marked for use with a resistor, that's only necessary if using breaker points.