The cost of using runflats is acrtually much greater to BMW, not a cost saving. The cost of a spare tyre is more than outweighed by the added development costs to have to design and manufacture a suspension system to handle both types of tyres, plus all cars with run-flats MUST have a pressure monitoring system otherwise you will not know you have a flat until the tyre disintegrates many miles later (as per previous post - how you know when you have a puncture). Another key factor is the weight reduction in not having to carry a spare and have somewhere to put it - this then helps with the fuel consumption and emissions reductions needed.
I may have mentioned this before on another run-flat discussion - in my driving lifetime I have had two tyres get an instant loss of pressure due to a massive puncture on the motorwat - the first normal tyre, the second a run-flat. With the normal tyre the first I knew was when the rear end suddenly pulled massively to the left - I tired to correct / catch it, but ended up spinning into the concrete central divider at around 70mph. With the run-flat I got the warning light on the dashboard, followed by a slight pull to the left, no spin, no massive accident. When I got home and checked the tyre it had a three inch gash from hitting some sharp debris on the road - no wonder it went down so quick. I know which I prefer to drive myself and my family around on.
As for not doing long drives on run-flats, I regularly do Auckland to Naper via Taupo and the Napier - Taupo road. On one Friday night trip I managed to hit a pot-hole in the road at speed, just past the cafe where it starts to get twisty. Massive bang and it felt like the strut had punched up through the tower! Slowed right down, and it still felt ok, sped back up to 80km/h for rest of journey. Checked damage in the morning and there was a massive bulge in the sidewall, and when I went to the tyre shop they found a massive buckle in the rim as well, fitted a replacement there and then no waiting. Pretty sure a non-run flat 40 profile tyre on a 19" rim would not have survived that impact and again would have been another blow-out situation.