There is one legitimate use for BitTorrent: to efficiently transfer data that you have permission to access.
Some people use lock picks to unlock doors that they don't have permission to much in the same way that some people use BitTorrent to transfer data they don't have permission to.
Fair enough. But even if I accept your definition of a "use," there are millions of files you could legitimately download with BT. There are a handful of locks you could legitimately open with a lockpick.
Some people use lock picks to unlock doors that they don't have permission to much in the same way that some people use BitTorrent to transfer data they don't have permission to.