I am surprised I haven't posted in this thread - but perhaps it is because it is so old.
Temporary spares (for cars) are designed to a standard with a lower margin of safety than the standard for regular car tires - as are the rims. That's the main reason for the warnings.
Further, temporary spares are manufactured to minimal quality standards - for obvious reasons.
While it it may be hard to believe, wider tires are frequently more efficient. Here's the background on that:
Barry's Tire Tech
Lastly, temporary spares are not designed nor manufactured with any consideration for fuel economy. If fuel economy is the goal, then a better bet would be to use a regular car tire that has been designed for fuel economy.