Carrying a boot is a great idea. I do that myself- usually an old gu packet and some dollar bills. They work fantastic in a pinch and will be much smaller/lighter than carrying a spare tire.

Of course, that won't help for every instance. Last year I had a blowout that blew a hole in the tire at the bead and bent the rim- so there was no fixing that. I *might* have been able to fix the rim if I'd had a screwdriver, but the tire was beyond repair since it blew out at the bead. Had to wait 3 hours on the side of a country road for my DH to get off work and come get me 40 miles from home. Whoopsie.
Lesson: find someone else to call in an emergency...ergo: I need to make more friends.