This is why you shouldn't put low tread tires on the rear of your MINI: A Cautionary Tale [as posted on MINI2]

I can speak first hand of this effect. The MINI being my first front wheel drive car I also rotated my tires last year. I figured I might as well try to get further wear out of the rear set that still looked fairly new. I went ahead and put my rather worn fronts on the rear thinking I needed more traction up front where my power was. It ended up making the car very twitchy and loose in tight corners. It was even worse in the wet. After a few weeks of enduring this setup I went to the TireRack to buy two more tires and was roundly scolded (in very nice way) for outting low tread tires on the back of a car like the MINI.

[Update] Just to clarify while rotating your tires every 3k + miles is safe and will make your tires last longer. However take note when you rotate your tires of how much tread you have left on the set going to the rear. If it's been around 10k miles or more you may be better off just buying two new tires.