I usually stay out of Car Tire debates but seeing that this discussion is not a heated argument I will add my personal reason for going to a car tire into the mix for what it is worth. It was not a decision I took lightly. I do not have any degree in engineering or any other technical info here and I doubt you will find any as no one in the field will want to say it is acceptable and have to be held responsible for what the general public does.

Along with my F6B and my Valk I have a 1978 GL1000 with a California Sidecar mounted. I will say the Sidecar is an accepted non OEM addition to my Old Wing but that Sidecar sure is a much larger alteration to my Motorcycle than the car tire is on my my bikes.
My point is that many of us make changes that an Insurance Co or Good Lawyer can exploit if push comes to shove in a court room. A chance we must except with our changes!!

My needs were for long trips
Flat repair. can be done to car tire much safer than Motorcycle tire
Better at heavy load handling
Better wet traction
No Cupping to deal with.. (a big problem with bike tires)

my Car Tire thoughts and experience ...............
we need to back up the calendar to 2012

For years I wanted to try a car tire on my Valk. It is a 2000 Interstate and as of late 2012 I had a bit under 90 thousand miles on the bike. The Valk is not my only bike, but the one that makes the larger trips as it is set up for long haul two up riding. I decided for this trip to take a friend up on his offer to throw a car tire on. He has 3 Valkyries all with Car Tires and had an extra mounted new tire on a Valk rim ready to go. I had done my researching and reading over the years and started adding up the pros and cons of a car tire on a road trip.

As in the past, our Sturgis trips take in 7000 miles in two weeks so this will be a great test.
I have found the extended highway speeds in the July/August heat, along with the bike tipping the scales at 1200lbs once loaded with gear and riding two up, sure does tear up tires fast. Sooo I mounted the NEXEN 205/60Ri6 tire. It performed well and I did not end up with a cupped rear tire as I always had.
I decided to stick with the car tire as my dedicated tire and not just for trips because I found no down side to the Car Tire.

NOW after running the CT for well over 40 thousand miles on two different bikes and many years time.... No change from Bike to Car tire in the twisties. This year to Sturgis we took the Northern route from New England thru Canada down into North Dakota. Three days of rain gave a good wet road test. Straight up stopping is awesome with the car tire, and even leaned over I still had good traction on the wet roads.
BUT I think the Bike tire has a bit better traction leaned over on dry roads. That may change with your car tire choice. The NEXEN I was running has very small tread blocks on the sides and I think the larger surface area of the bike tire gives it an edge . Although I never felt any slipping until I was accelerating a bit harder that I should have while leaning in a hard corner up hill...

PRO...
Run Flat tires (if you so choose) I have had both
Flat repair much easier and can be run with a plug
Better at heavy load handling
Better wet traction Much better tread life
No Cupping to deal with.. (a big problem with bike tires)
On the Bearthooth Pass we ran into construction and rode 10 miles of dirt in the rain... I was glad I was on a Car Tire for that!!!

Con...
Low speed handling on uneven pavement
Motorcycle dealers don't like to deal with them (but small tire shops have no problem)