A space-saver spare tyre is:
(a) a get-you-to-the-tyre-place remedy
(b) a good substitute for a blown tyre at motorway speeds
(c) a good substitute for three blown tyres at motorway speeds
(d) a good substitute for a missing 21" hoop
If you said (a) then you have a brain, but sadly, the other three answers appear to be the most commonplace now. I see people flying along the motorways at 80mph or so every day with space-saver wheels on their cars. I've also seen cars on the motorway with three of the four wheels replaced with space-savers, and you guessed it, I've seen people with tricked out cars with big wheels missing a wheel but using a space-saver (meaning one wheel is radically smaller than the other three).
It really baffles me how people's brains can malfunction so badly that they don't understand how dangerous this is. (Once again, I believe that you should understand everything about your vehicle if you are to be allowed to drive it.) Most drivers will never have to use the spare that comes in their car, meaning most of us pay it little attention. The rubber sits there under the floor, perishing slowly, leaking air and when the time comes that we actually need to use it, it's all but useless. But for some people, they see it as the perfect replacement. It's not. The rubber isn't designed for heavy loads and high speeds - it's narrower than all your other tyres and doesn't have the same strength or number of steel belts and cords in it. The tread is generic, so it's no good in the rain at anything other than walking pace, and the wheel itself is pretty flimsy, lightweight steel, not the decent load-bearing stuff your normal wheels are made of. Space-savers should only be used as the manufacturer intended. If you zip along at 80mph on the motorway on a stormy day with one of these things, you're pretty much going to get what you deserve. The only problem is that your stupidity will likely cost someone else dearly at the same time.
There's no excuse for doing this. If you have to use one of these things, don't drive home with it - drive to the nearest tyre change place and just get the damn thing fixed.