Despite the fact that nobody cites any evidence to prove that there are more crashes because of the extra power, that's still the assumption. If we were all forced to ride Yamaha Jogs and Rebel 250s, we would probably still see similar crash rates, although there would admittedly be fewer fatalities and less-interesting crash stories. We all want to reduce fatalities, but the best protection for a motorcyclist is to avoid crashing in the first place. If a rider is untrained, will putting her on a smaller bike really make her less likely to crash? Since I have no idea how to fly an airplane, I'll crash an ultralight just as fast as I'll crash an F-117, although the F-117 crash would probably create a more dramatic explosion.
Like helmets, tiered licensing wouldn't lessen the number of crashes. Instead, it would merely mitigate the effects. If motorcyclists could somehow muster the discipline to not drink and ride as well as attend the MSF course before they purchase their dream machines, the crash rate -- as well as the numbers of fatalities -- would be so much lower that helmet laws wouldn't even be an issue. The number of lives saved by helmets in a state like Washington would be measured in dozens rather than hundreds. More people than that are killed hitting their heads after slipping in the shower.