That's only half the reason, though you do notice a distinct RPM drop when only one set is operating. The main reason, though, is redundancy
I used to think that too, until I started doing accident investigations. To be accurate: the reason for the twin magnetos is redundancy, sort of. Some "dual" magnetos are actually in one unit, and many of them are on a common drive that is much more likely to fail than an individual mag. So, not so redundant, but better than just one. Mags rarely fail.
As for the plugs, the FAA test question answer is for more efficient fuel burn. Remember, those are BIG cylinders with relatively low compression. Not to mention the massive electrode REM 40 E's that most Lyc's and Continentals use. As for the RPM drop, be really careful if your renting airplanes that do this. The maximum drop in most applications is 50 rpm. That's the WORST case. If the mechanic has timed the mags properly, there should be only a slight drop, less than 25. If you get a differential of more than 75, don't fly it. There is a timing issue that will bite you some day. The Barons I fly regularly have an rpm drop of about 10( a needle width). That's because they know how to time the mags properly and use the proper equipment, rather than just "by ear", which I see all the time.
If you fly, please be careful, compulsive and constantly aware. I am getting tired of scooping bodies out of crashed GA airplanes, and it's getting worse.
OOOps, I'm sorry. This is the INSIGHT forum. My apologies.