Aside from the fact I already occasionally narrate my own life and talk to myself a lot, it just seems pointless. Also, a problem occurred to me. Well, two problems. First, as somebody else already mentioned, is masturbation. Depending on how graphic the narrator gets, I may never want to masturbate again.
Second, what about sleeping? If the narrator also narrates what you think, does he/she narrate your dreams as well? I see this becoming a problem with actually getting enough sleep to function. In short, a narrator might be a cool idea for the great moments in life, but just seems impractical the rest of the time.