We managed to pick up half a pair of scissors when towing in France with our BMW last summer. (It was still in the tyre when they took it off so I kept it as a souvenir). The car was fitted with wide low profile tyres, different width front and rear and a space saver spare.
We were in Cahors, a fair way down France and on our way home when it happened. I decided that with a fully loaded car and towing, I would not tow home with the space saver. Although my tyres are Michelin (French) and a fairly common size in UK, they were virtually unobtainable in France. I ended up going round several tyre places (Red Pennant were very helpful but had little luck also). Eventually we found a pair of Chinese tyres which were the right size and they got us home, but it still delayed our ferry crossing and return to work by a couple of days and it was fairly hairy at the time trying to locate the tyres.
For subsequent trips last year, I bought an extra front wheel off ebay and a new tyre. It just fitted in the spare wheel. A bit of research showed that the rolling radius of the wheel tyre combinations was the same front and rear although the widths were different. I reckoned that if I had a puncture on the rear, the spare front wheel was better than the space saver, buts its probably still dodgy.
I would certainly think twice now about towing, particularly abroad without a full sized spare. I think the same may apply to run flats. Many cars supplied with a space saver do have room for a full sized wheel, and my mix of tyre sizes isn't common. Buying a second hand wheel as a spare is an option in these cases.
I think VW and Audi tend to still use full sized spares. We are now towing with a Series II Disco, which has a full size spare, and more importantly loads of room for bringing wine back!
Regards
Tim