Ralf S., do I recognise you from the FIAT Forum?
Spider drivers, Let's be fair about this: you can't carry a suitcase in the Spider (or a box of anything purchased) without removing the spare wheel. Then, the spare wheel just won't quite fit behind the passenger's seat (it slips down and prevents the door closing). And in the event that you did use the spare wheel, where are you going to put the removed full-size wheel if there's anything else in the boot?
In the last 14 years I've had four punctures while driving. Three of those were in two weeks driving my 156 with 16" wheels (three different tyres). One was a metal hook that fell off under the car the day I bought it. Another tyre developed a puncture 10km down the road while I was trying to find somewhere to get the first puncture fixed, so even with a spare wheel fitted, I still had to drive with one flat back tyre. It seemed to be a failure of a previous plug-type repair.
Apart from the metal hook, the other three punctures have been of the no-obvious damage variety that sealant would probably fix. Even if there is a screw in the tread, sealant would probably fix. I don't like driving on repaired tyres so if it's punctured, it's probably destined for the silage heap, therefore the mess made by sealant won't matter.
Going back further, the first while-driving puncture I ever had (FIAT Uno) was on a rear wheel and I drove 50km without noticing the 135/80/13 tyre was completely flat.
The sidewalls were worn smooth, doing a remarkable job (low profile tyres are better at this).
I've had three other tyres over the years with the mysterious go-flat-one-night problem, which is fixed by replacing the valve (split rubber) and/or refitting the tyre. These at-home punctures don't require a spare as I leave the car on the jack and put the wheel in another car (to take the tyre for repair).
My Spider has the spare fitted at the moment, but I intend to remove it. If I get a puncture, I will put the punctured tyre on the back (will jack up one side of the car 2/3rds forward so that both wheels come off ground together) and I will put in some gunk and reinflate with compressor which I carry (cheap to buy those). It fits nicely in the space beside the jack, on the shelf in the Spider boot. In the worst case scenario this means I'll be driving with a flat back tyre, which I have done before.
Four punctures during trips in 14 years and two of those at the same time - I think there is a greater chance of a big-end failure or cambelt failure than there is of getting a puncture.