True it is variable, but current flow has everything to do with charging the coil. The higher the current the faster it charges. Go stick that 5ohm ballast in place and see what the current flow drops too. See the effect on the spark intensity. Charging or energy absorption of the coil is current and time dependent.

The variable nature of the ballast was designed to prevent over heating of the coil at low rpm when dwell time is the greatest, hence higher resistance and less current. At high rpm you want more current flow because you have less time between firing events to charge the coil.

I was keeping it simple to explain the ECU. It is much more involved than a simple on off switch.

Regardless, change any value of a resistance, or voltage drop in a simple DC series circuit, and you change the current throughout the circuit. The ballast can be in front or after the coil, and it would have the same effect.

Bosch has a good tutorial on the coil and charging on the web.