Safety and gearboxes
It is well known that Swedish engineering is a leader in car safety development. Trollhättan is located only two hours from Göteborg (Volvo).
IMHO, I would think BMW is looking for two things in SAAB: safety technology and gearboxes. GM's transmission development has at times been almost led by SAAB, and patents of how to integrate different propulsion sources in one vehicle (electric) as well as 4-cyl turbo charged gasoline engine technology.
Hope they know what they are getting 'emselves into..