According the BMW M-Series..
"In 1989, Motorsport engineers developed an M5 convertible, with two lengthened front doors and seating for four. It really did come very close to production - a price of (pound symbol) 50,000 was agreed, and space was even booked at the Geneva motor show, but one week before its intended debut, BMW 'killed' it, believing it would have led to a demand for non-M 5 Series convertibles, which might have dented drop-top 3 Series sales."
That M5 convertible has M System 2 (throwing star) wheels. I personally like turbines. Maybe BMW put turbines on the 3.6 M5 to make sure the brakes would not warp like the E28 M5. I think there are other BMW wheels that are "uglier" than those. For example, the 15 inch wheels for the E46 with all those holes.
If this M5 convertible led some within BMW to think it would have (and it likely would have) dented E30 convertible sales, then why didn't anyone stand up to the E36 M3 sedan??
Why not come out with it when the M3 Evo arrived and the M5 was gone?
That way, there would have been more M5s to love.