I got the CI three years ago, and was talked into getting both a BTE and a body worn. I guess it really depends on where you go for your CI, what brand you are getting, personal choice, etc....
I NEVER use the Body worn processor, the only time I do is when I go to get Mapped and then I just bring it along. In the beginning I was "talked into" getting a BWP besides a BTE because they (my audis) were convinced that people did better with the BWP in the beginning than the BTE. I guess it is mostly because with the BWP (Nucleus) they could download 4 speech strategies into the BWP, where back then they could only download 2 programs into the BTE, and they wanted to give newbie CI users more options with the 4 programs in the beginning because there are so many different things that you need to try out when you first get the CI.
However, I personally wish I didn't get the BWP, because I never wear it. I ALWAYS wear my BTE cuz it is so darn comfy and not a hinderance. My BWP just sits in a drawer at home gathering dust. I might bring it back to my CI center and see if I can trade it in for a second BTE (for backup).
Now with the new Freedom (Nucleus) implant I hear most people just get the BTE, cuz the new Freedom has tons of features that the older model (Nucleus 22 or 24) BTE's don't have, and you can also download 4 programs onto the Freedom BTE..