I think they introduced the Cayman in order to have only one "base" 911 in the future, and not 2, like there are now (the 997 and the 997S). The 998 will be the successor of the current 997S and there won't be a successor to the 997, which place will be replaced by the Cayman series. This is only speculation of course!
All this will result in:
- the 911 being more expensive, hence more exclusive,
- the Cayman, although a car of its own, will nevertheless be the entry coupe for those who might eventually upgrade to a 911 later on.
Porsche is aiming a younger audience with the Cayman and, like RC already said, Porsche needs to make money! And a Cayman sells better than an exclusive, expensive sportscar...