My theory is that after the initial outlay, having both winter and summer tyres will not cost any extra money. I am hoping that I will get three winters out of my new set of winter tyres. In the time that they are on, I will not be suffering increased wear throughout the winter on sumer tyres, therefore increasing the longevity of them.
I can only wear out one set of tyres at a time, so it makes sence to use the best performing tyres with the least wear rate for the appropriate season. The savings in tyre wear, should pay for having the tyres swapped over come spring time and then next winter.