I was at Hilton Head, SC, recently and took a beach bike from my sister's vacation home to the LBS for replacement of the tire, which was worn and flat, and the LBS guy highly recommended the thick thorn proof innertubes. He put them on all his rentals, as this fixes the issue of customers getting flats. That said, it didn't look like he uses kevlar tires. I inferred that thorn proof tubes were cheaper than kevlar belted tires.

That said, given the weight of such a tube, which comes in a box that's double the size of a regular tube box, I'm certain that it adds more weight than the belted tires.

I recently got a flat on my bike, which I use Schwalbe Marathon Plus's, which shocked the heck out of me and ended up being the first time in all the years of bike commuting that I had to ask for a ride home. The tire wasn't the problem--it was a tube defect near the valve stem that bubbled up and a patch wouldn't hold. I didn't have a spare tube--just my patch kit. I later replaced the tube with a thick, thorn proof tube.

Double protection, with kevlar belt and the thick tube, is nice!