I can see why they would be better on the front but.............. Space savers are thin, but not mormally a different diameter from the standard wheel are they? I can't say I've ever notices a car on a space saver looking dipped down in one corner as surely this would be a really big safety issue, more so than having a thinner go slow spare tyre.
How would a diff ratio get "kncoked out"? A diff is designed to spin a different speeds on either side all of the time, and it is happening 99.9% of the time (as very few roads are dead straight) so are we all driving around on diffs with knocked out ratios??
It is only the width that is different on my car. The skinny can be fitted to any corner, but there is a clear recommendation to run it for as short a distance as possible and with a maximum speed of not more than 50 m.p.h.
The confusion about fitting it only on the front arises because that is what Lexus recommend when there is snow or ice; but they also recommend fitting snow chains to the rear in these circumstances. Personally, if I got a puncture with significant snow on the ground, I would be calling the RAC . . . . . :P