I suspect the logic was that it was cheaper, if you had the router already, to buy the biscuit joiner attachment than it would be to buy a biscuit joiner. Lots of tool accessories make sense only if you assume people are interested in saving money, even at the cost of some inconvenience.
But hey: at a minimum you wound up with one of the better routers ever made. If I hadn't stumbled into Porter-Cable routers as my standard, I'd probably lean toward the classic B&D routers.