I've been following this story for quite some time and it makes a lot of sense for both parties. Merging the maufacturing operations ensures cost and production controls for the parent company, Porsche SE (holding company owned by the Porsche and Piech families).
Porsche SE had to leverage to purchase the VW shares and a merger of the manufacturing operation could reduce the parent's debt substantially.
http://www.bloomberg.com/apps/news?pid=20601085&sid=atigQAhcVHsI&refer=europe