Whales are mammals. Their ancestors lived on land and therefore had legs. The remnant hind legs are a throwback to these evolutionary predecessors. My question was therefore to assertain why an intelligent designer would put such redundant bone structures in an animal.
It wasn't redundant while on land, though. I think you're falling into the trap of thinking every creature should be perfect and static in its environment.
The astonishing thing, really, is that a land mammal can spend time in the water, and after a period of time, magically develop all the equipment required to live successfully under water!