actual need for ballast resistor with electronic ignition?

First, MSD has zero DC current through the coil. Completely different.

I would bet that modern E core coils have a completely different current curve than older "can" coils

"Current control" is surely the reason. It is called a ballast. it is not a plain resistor. Anything termed ballast is designed to stabilize and somewhat regulate current.

The only thing that I find odd about the Mopar system is the ign1 / ign2 circuit, BECAUSE, during start, you want a hot spark, and YES, the IGN2, JUST LIKE with breaker points, runs full battery voltage to the coil. HOWEVER, during cranking/ start, the ECU ONLY GETS A PIDDLY voltage backwards through the ballast!!! I have never figured out how that could work well, in a bad situation say, cold weather and a low battery.