[BillCB]

Not entirely true - if you get a slow leak (from a nail for example) but keep adding air so the sidewall doesn't collapse, the tire can be repaired. Only if you are forced to drive on a tire that is truly flat will you "crack" the sidewall so the tire cannot be reused.

I speak from recent experience with a roofing nail. I did NOT remove the nail when I noticed the tire was soft but put about 45 lbs pressure in the tire before driving to the dealer, adding air every 50 kms or so. (It was a 125 km trip.)

There was no evidence of sidewall damage because the tire pressure never got below about 20 lbs.