Money has little to do with it for me, I have roughly 100k on car tires on 3 different bikes. 98 Valk, 12 Wing and my F6. I have both a MC tire and car tire mounted and use both.
Benefits for me include:
Wet road traction- I live in Florida
Load capacity- I have gone cross country with the wife on the back pulling my 16 cubic ft tag along with all of HER stuff in it.
Run Flat-Had a Flat at work and road home 6 miles with zero pressure. Made me feel good later riding out west alone knowing I did not have to worry about a flat in places like Death Valley.
Panic Stop-practice them occasionally, might just be me but can stop quicker with the Michelin Alpin on the back.
Ride-Wife claims the ride from the back is smoother and quieter than with the OEM (which whines after a few thousand miles)
Last of all is mileage-not about $ though. I have Summer's off and have taken long trips (such as 12k in 6 weeks Florida to Alaska and back) It sucks having to have a tire change during a trip. Fronts last longer on my bikes and the car tire last much longer.
Add all of these up and the fact that I can hardly tell the difference and it makes sense to me.
It really boils down to a peace of mind. I like to be comfortable on my bike and I Am with either tire so I can see no reason not to use a car tire when I want. To each his own!!