What else?
Supercars are OK for the sports cars brands like Porsche, Ferrari, Lamborghini, Aston Martin etc - since such cars are natural extension of the existing model line of sports performance cars. Or for supercar brands like Pagani.
All the others are producing supercars :
a) to show the world they can do it (eg. Toyota, Nissan etc)
b) to improve/polish the broken image (eg. Mercedes-Benz)
c) to build an image of a performance brand (eg. Lexus, Audi, Acura)
Usually carmakers use an excuse of their supercar being a celebration of their Motorsport success / or only a celebration of participation in Motorsport.
Mind that R&D costs are usually enormous, and such cars are sold in small quantities, or even as limited editions. So they are not profitable, or very marginally profitable. Usually such cars are used as guinea pigs for new technologies ("tour-de-tech" cars), and more even important: for marketing reasons - as instant image boosters.
At this moment BMW do not need image boost - so they are saving supecar for emergency cases, for the times image boost will be needed. I hope this time will never come.
Like Scott said: all BMW focus is directed into Z8 & 8er successors - also as ultimate M versions. M GmbH is also occupied with CSL models, so ...
New "Z8" & new "8er" (CS Concept based coupe-sedan) will be potent enough, and will be more appropriate new models for BMW than a special supercar model. Also for aficionados there are CSL models. And there will be more of them in the future.