There is no law or rule that says if your car comes fitted with run flat tyres you must replace them with run flats. In fact, many owners remove run flats as they find they don't perform as well as regular tyres, with higher noise levels, a firmer ride and worse grip. Of course, the only thing to bear in mind is you'll then have to rely on a can of foam sealant to get you home, or stick a spare in the boot.