Two magnetos were necessary for safety because magnetos are notoriously unreliable. They're a lot better than they were but they still fail. The manufacturers of magnetos stipulate 500-hour inspections (Slick) or 400-hour (Bendix) to catch the stuff that's wearing or corroding or whatever, but since FAR 43 is so soft on a lot of stuff it seldom gets done and owners run their magnetos until they fail. To me, that's just plain stupid. An owner will carefully preflight his airplane, making sure the oil and fuel are up and there's no bird nest under the cowl and that the runup goes well, but they'll run the mags, vacuum pumps and alternators until they quit. And they usually quit at some inconvenient or dangerous place or time. A magneto can do more than quit, too: it can wear the plastic gears in its distributor so that they slip and the thing starts sending sparks to the wrong cylinders at bad times such as the intake stroke, making the engine run so rough it just about quits. The mag switch is there to shut that bad mag off, but I once watched an RV trying to make it to the airport with the engine barking and coughing from a bad mag. I presume he made it, since there was no news about any accident.
Poor training, there. How many of you were ever taught to try cycling the mag switch if the engine started running real rough and carb heat didn't fix it?
New engines are now certified under revised FAR 33. In the Appendix regarding continued airworthiness we find this:
The Instructions for Continued Airworthiness must contain the following manuals or sections, as appropriate, and information:
<snip>
(6) Scheduling information for each part of the engine that provides the recommended periods at which it should be cleaned, inspected, adjusted, tested, and lubricated, and the degree of inspection the applicable wear tolerances, and work recommended at these periods. However, the applicant may refer to an accessory, instrument, or equipment manufacturer as the source of this information if the applicant shows that the item has an exceptionally high degree of complexity requiring specialized maintenance techniques, test equipment, or expertise. The recommended overhaul periods and necessary cross references to the Airworthiness Limitations section of the manual must also be included. In addition, the applicant must include an inspection program that includes the frequency and extent of the inspections necessary to provide for the continued airworthiness of the engine.
See that reference to Airworthiness Limitations? That makes the inspections mandatory by law. No more running stuff to failure on new engine designs.