boraticus this was actually for emissions from my research on the ford 2.3l. The plugs on the exhaust side fired on the exhaust stroke, the haynes or chiltons manual stated the reason was to burn off any leftover combustion products. One more note if you unplugged a wire from the intake side which was the main plugs that fired of compression stroke the engine would run as normal, you could tell no difference.
However on the lawn engines I do agree it's a cheaper method, simpler, and for us way less of a headache to deal with than some mechanical (lovely points that everyone loathes ..I hate them myself, however I have them on my old Kohlers and seldom do they give any problem, or wear out as you hear read so much about)or even electrical device to provide spark on power stroke only. ( Compression/power would be very close LOL). I did not know this information on small engines.
Key note on points above, if you have the switch jerry rigged it will burn the points if fire is on them in closed position, and from what I have seen there are huge amounts of electrical engineers in the lawn mower business.. clothes hanger, tape if you name it.