Only just spotted this, but in the UK a lot of both E70 & E71 owners are ditching RFT for normal tyres on the 20" wheels because of the improved ride comfort. The only requirement owners are advised to meet is to tell our insurer because as the cars aren't offered in our market without RFT, changing counts as a 'mod' and needs to be notified so as to avoid the insurers walking away in the event of an incident.

BMW GB also refuse to warranty the transmission if there are issues, as the tyres are now non-standard.

I can't see any engineering reasons why you would not be fine with a pair of each, as long as you have something to cover you in the event of a flat.

__________________
Me:
Current: 2011 E71 40D Vermilion Red/Beige Nappa

Wifey:
Current: 2012 R80 Countryman SDX Green/Cream