A couple of thoughts:
Donut spares are designed to go 50 miles at 50 mph. That does 2 things:
If you are familiar with the fatigue of materials, the number of cycles to failure can be described by the S-N curve for that material. All structural materials - including rubber - have similar S-N curves in that the more cycles that material has to endure, the stronger the part has to be. Put a different way, if a tire is designed to go 50 miles, it can be built to a lower strength level than a tire built to go 50,000 miles.
Ref:
http://en.wikipedia.org/wiki/Fatigue_(material)
Also, the higher the speed capability, the higher the strength of the tire has to be. 50 mph tires are built weaker than 112 mph tires.
The net effect is that the government standards for Donut spares is quite a bit lower than for normal tires. There are no wear requirements. There are no traction requirements.
Donut spares are also manufactured to lower quality standards, since everyone in manufacturing knows what their intended purpose is.
Overall, using Donut spares for purposes other than what they were intended for involves some risks that normal tires do not have.