I have been driving an XF Roadster for three years now. Total of just over 50,000 miles (DAMN!) and only just recently did I have a leaky tire. I'd picked up a small sharp piece of metal in the parking lot at work (I am SURE that is where it came from).

I nursed it along for about two weeks by putting air in every three days - then FINALLY got wth the program and took the tire off hte car, found the offending intruder, and pulled it out with needle nose pliers - which caused air to escape at an alarming rate! I used what I call a "Tire Worm" and rubber cement and plugged the tire in about a minute. I pumped it up to 35 psi and have not touched it since, about a MONTH now.

I carry a few tools and a "worm kit" in the trunk - I have had a tire blow out only ONCE in 33 years of driving, when I hit a big piece of metal in the road.

After 33 years and good luck with "Tire Worms", I have no need to carry a spare. I pride myself on not having a spare and still taking off across the country when I want to.
If I get stranded, then I guess it'll cost me a night on the road - no big deal.