I've never ridden any darkside motorcycle, so I can't weigh in on this debate. Further, I've never heard of any crash or failure caused by darksiding, so it seems more of a personal preference. However, the Tire and Rim Association publishes specs on, surprisingly, tires and rims, by which all tire and rim manufacturers abide. These ensure that car tire bead profiles match car rim profiles, and motorcycle tire bead profiles match motorcycle rim profiles. We can all agree that they're different. Whether or not it's "safe enough" is up to the individual rider. Motorcycling is factually many times more dangerous than driving, so we're all already taking a verifiable risk every time we ride. Is there any actual evidence of increased catastrophic tire failures with car tires over motorcycle tires on motorcycle rims?
My point is, what's the big deal if someone wants to darkside and doesn't mind the handling change? Every aspect of motorcycle design is a compromise on handling, power, emissions, comfort, cost, reliability, weight, etc., and this just seems like a rider customizing their ride to their preferences on those scales. One person's compromise is clearly not another's, but neither is "better".
Examining the “Dark Side”