According to the laws, you are only permitted to employ a spacesaver on the wheels of the vehicle's rear axle. Two main causes.
a) Since the front axles are the driving axles, more force is transmitted through the smaller profile tyre than it was designed to handle. This might cause the tyre to wear out prematurely. On the back end, they are barely lagging behind.
b) It should come as no surprise that the steering is done using the front axle. Therefore, for the same reasons as stated before. More twisting force than the tyre was intended to handle.
For the same reasons, there should also be a third limitation placed on the vehicle, which is that the maximum speed should be capped at 80 kilometres per hour (50 miles per hour). They are only intended for use in an emergency to get you home or to the closest tyre provider as quickly as possible.
But try explaining it to some of the individuals you meet every day who are still operating on a screensaver. They won't believe you.