Generally when you make the ball and the shifter assembly two seperate pieces it's to save money on the materials used to make it (billet aluminum usually). This way instead of using, say, a 2" thick block of aluminum, you can use a 1" thick block that's just a little bit longer, machine the ball on the end and then weld it.
:shrug:
Personally, I'd just save money and get the B&M.