Ballast resistor Q's

so enlighten me please .... why can I delete the ballast and run battery voltage to the coil of a pertronixs ignitor 2 w/ matching coil, but not a mopar performance ignition with non-factory coil ? ... I'm confused as to why I would need the ballast if I no longer have points or the factory style low voltage coil ......Thanks (oh by the way, I know running an msd coil is not an msd ignition system )