I believe the tires are a bit hard to come by, but they are by no means a dealer only item. They are also not a unit with the wheel.
If you have to use the tire sealant kit that comes with the car, it will get you where you need to go, but the tire is done for. Patching or plugging is an option, but only if you don't use the goop that comes with the car.
I think that I overheard that the plan can only be bought at the time of purchase when I was at my dealer last.
I look at it this way (knocking on wood). I've got close to 300K in driving under my belt in the past 10 years, not counting the 237K I racked up as a bus driver in college. In that time, I have never had a flat... I've had a few screws in my tires, but never a flat. For the cost of this plan, I can replace 2 tires. By the time I have to do that, they will all be worn out and need replacing.
Same thing goes with wheels... it's not like these are 12" Geo Metro wheels. Destroying one would suck, but it's a 15" wheel with plenty of padding... I doubt I would kill one in 5 years of driving, I dunno about you.
If you can't tell, I am not going to be buying this. I'm still on the fence about extending the warranty for another year though.