I found a response to your question on Popular Mechanics website. I think it gives some pretty good reasoning:
First, a temporary spare isn't as durable as a normal tire. The real strength of a tire comes from the plies—layers of steel and polyester underneath the rubber—and spares don't have as many plies as regular tires. A typical space-saver spare has only one layer of polyester in the sidewall and two belts of steel with a layer of polyester in the tread—about half as many plies as a normal tire. This greatly limits puncture resistance and cornering ability.
It goes on to state:
... these tires are narrower and have a smaller contact patch. This reduces the amount of traction for the tire, increasing stopping distances and making handling potentially unpredictable in emergency maneuvers. It also means ABS and traction control aren't as effective at keeping you out of danger. And you're not going to have the same ground clearance.
Plus it goes on to talk about the differential if you are using a doughnut on the drive axle:
Because the spare is smaller than the opposing wheel on the same axle, it must turn faster to keep up with the speed of the car, making the differential work to account for the variation. It's as if the car is constantly in a turn. Leave the spare on long enough and the grease lubricating the differential will begin to break down, accelerating wear between the gears and the clutch plates if it's a limited-slip differential.
All this adds up to one thing: utilize the spare as a means to get you to the shop to get your tire repaired or replaced. It doesn't do you any good to trust it further.