Unless owner of said bike can be absolutely certain of never riding in inclement conditions, on twisty roads or requiring any kind of evasive manoeuvre. (But that never happens.) And if he can get his insurance company to state in writing that they're ok with it. (Best of luck with that.)
Makes you wonder why owner of said bike would even want to ride.
Went on a ride today and there was a Vulcan with a car tire. He stayed with us on the curves. Slow turns in the parking lot were the only issue. Not sure what inclement weather has to do with it, cars handle fine in the rain and there should be no difference with a bike.
As for why they ride, if you have to ask....