I disagree...
When my runflats on the front wore out on my old S I got a set of Eagle F1's fitted to replace them, but I put the grippier F1's on the rear and moved the good runflats to the front. I drove on this tyre combo for about 6 months and had no problems with grip or control, and you know how I threw that car about
I see no problem with it as long as you move the RF's to the front.