A couple of points here:
If you buy wheels for you winter tires you will actually save money in the long run. If you run winter tires and summer tires on the same wheel you pay remounts two times a year. Over a few years the wheels will pay for themselves. Plus you will not have to sit in some tire store while they do the swap. Also less likely you will have to deal with balance issues if you are not remounting two times a year. One drawback for many on this forum is they have nowhere to store tires or do the changeover.
Now the run flats are slippery even on day roads when the temps drop below 40 deg C. so itís not just the snow you have to worry about.
Say you stay on summer tires and have an accident. Say you have $500 deductible and would have to pay the $500 which could have gone toward the winter tire setup which may have helped you avoid an accident.
Just my 2 cents but not only my 135i gets a winter set up, I have a Toyota Tacoma 4x4 with locking diffs that I run winter wheels that are sipped and studded. No way am I going to miss a powder day on the slopes! I have had a few near misses on snow where people out of control and I was able to avoid the situation due to the control on snow with a winter setup. No Brainer for me!
2015 M235i - Estoril Blue - 6 speed - Black Lather - Cold Weather Package - Premium Package - Harmon/Kardon Premium sound