OK you have all 5 seats occupied and the boots full, Sod's law you get a puncture, you go to the boot empty it get your space saver out and replace your punctured tyre, now were do you now put that 20" x 8" wheel
Slide the back forward if it's not all ready forward to make more room in the boot. Less room for the passengers in the back. One things for sure them wheels are so expensive to buy you wont be leaving one of them on the side of the road
As the tyres are self sealing apparently 80% of punctures shouldn't present a problem so there would not be the need to use the space saver!.....the space saver is there for the other or 'unlucky' 20% where the side wall is damaged or the tyre is beyond repair.