Spark coils don't like too much voltage, so most 12V or ballast-
less 6V coils do have the extra resistance, but it's built into the coil. There is no advantage to running a 12V coil rather than a 6V with a ballast. Some cars use a resistance wire that acts as a ballast resistor woven into the wiring harness, which can be real confusing if you are doing an engine or harness transplant and leave it out.
BTW the fact that a ballast changes resistance as it heats explains why some tractors will start immediately or not at all - the heating of the ballast resistor combined with the drop in battery voltage can quickly degrade the voltage to the coil down to the point where it can't produce an adequate spark. This can be a special problem when for some reason the coil is overballasted so it is putting out a marginal spark in the first place. So, if one is playing with non-standard coils or ballasts it can really pay to make the extra effort to get the ballast resistance just right.