I digress a little but speaking of 3 series exclusivity, BMW produces
18 variants of the 3 series in various markets (according to Jeremy Cato of Car Business/Globe & Mail journalist in a recent interview). Lending engine technology to others is something BMW is acutely experienced with - I fondly recall the BMW sourced 4.4 L V8 power plant in the 2002 to 2005 Range Rover's.
And speaking of partnerships, check out this article - mind boggling to say the least...
BMW to save billions by 2012, with the help of Mercedes-Benz