I am starting to do my CPL for several reasons...
First as I have not done a lot of flying these last years (though I am in the process of addressing that), I find the training very helpful in keeping not just proficient, but focused on flying and some of the more tedious parts of theory one would not normally keep current on after you get your ppl, and to be in a situation with interaction with professional pilots to learn from their (sage) advice.
As a renter having a commercial license helps with certain insurance problems, from what aircraft you can hire, particularly turbines, limits on what you can do (the whole reason I got my helicopter license was so that I did not have to land on a 10,000' runway) and hopefully if/when I buy to reduce my insurance premiums.
Oh, and I also want to wear those fancy gold bars on my polo shirt
