First of all, one must ask themselves why they wish to go with an Iridium plug on a scooter's motor.
Easy question to answer.
Firstly - when any given spark plug gets worn, the spark plug loses efficiency, the fuel does not burn properly, unburnt fuel start passing the piston rings, an the unburnt fuel ends up polluting and thinning the engine oil. This obviously means more wear on engine components will start taking place - simply because the spark plug is worn.
Now who on earth would like that - even on a state of art scooter engine ?
Secondly - when any given spark plug start losing efficiency, the engine obviously start losing efficiency as well. That's when it becomes "normal" to open the throttle - when owners try to "force" more power from the engine.
In real life, opening the throttle more than usual only means more unburnt fuel will end up inside the oil sump, and wear on the engine will start taking place even faster.
Thirdly - iridium spark plugs are not that expensive, and will easily outlast the lifespan of most scooter engines. Even on smaller scooters it makes absolute sense to install iridium plugs - simply due to reasons mentioned above.
I like fiddling on mechanical things, but simply fail to find any good reason why iridium plugs should NOT be used on any of these scooters. Specially on more expensive scooter models where owners prefer to keep scooters for a long period, I think it makes absolute financial sense to install iridium plugs on their scooters.
After all - who in his/her right mind would like to remove those panels on more expensive scooters just to replace a spark plug ?
Just my 2 cents.