Alright.....really dumb question: Is darksiding better if done on heavier bikes? Does the weight of the bike make a difference?
Usually, the heavier bikes are touring bikes ( but not always). So heavy bikes like a Honda Goldwing, Kaw Nomad, Vision, XC, or XCT are heavy and may travel alot on the slab or freeways. I think my twisty stuff has to be around 10% of my total miles (2011 XC/14K miles).
In addition, a motorcycle is using one of the smallest car tire sizes out there, they don't get much smaller than 175/60-16, 195/55-16 and so on. It becomes a question of "will it fit?". Some car tires of the same size still vary in width and height with the same size numbers.
Another thought to consider is wet weather traction is much improved with a car tire, especially the all-season kinds, or softer snow-type treads. My Michelin Primacy Alpin PA3 ZP (run flat) 195/55-16 throws out a heck of a lot of water in the rain. Some other bikers won't ride behind me in the rain because of this water dispersion deal. For me, I like the wet grip. Fortunately, it even works better in some snow - which happens at odd times in the mountains.
My Michelin doesn't care about tar snakes. The original Dunlop E3s are terrified of tar snakes. Works for me.
If you are going on a long trip, say 5-6000 miles, it is a pain to take off a perfectly pretty-good tire to make it all the way through the trip. Of equal annoyance would be having to get another tire somewhere out on the trip. Add mileage and Run-Flat safety are pluses for the darkside in my book.
The darkside is not for all bikes or even all riders of heavy bikes. You make your own choices and go from there.