In my opinion, there is no advantage to using rare-earth-electrode plugs.  The typical methodology is to change the electronic-ignition plugs every annual or 100 hours, where aircraft-type plugs are generally never changed, except on condition.  There are two reasons for this - the automotive plugs used in the(most, anyway) electronic systems are cheap and they have less clearance between the insulator and housing and therefore are more prone to suffer from lead fouling.  The reason aircraft plugs are so large is to have a large enough gap to allow lead buildup, and to allow easy cleaning.  If you are going to throw the plugs away after 100 hours of use the extra durability of the iridium plugs won't buy you anything.  I would not spend the money.