Magnetos use the rotational timing of the engine and crank itself to switch power from a coil to the sparkplug. Magnetos are very reliable, but most aren't adjustable (for advance and the such). Magnetos also produce a very powerful arc that interferes with radio and just about all transmitting electronics. No one around you can hear radio, talk on the phone, etc.
For planes this isn't a problem. The Arc is far below the amplitude of FCC radio frequencies, and the reliability of it outweighs the problems. Also, lots of airplane ignitions have individual coils just before each sparkplug. That means that the RF bleed is rampant.
-> Steve