Bananas are one of the most accessible types of fruit in Japan. They're cheap and you can buy them nearly anywhere including conveninece stores. Of course, they tend to not be very tasty or sweet compared to some of the bananas you can get in other places (at least the Tokyo ones). I'm guessing this Starbucks concoction is going to be quite a bit sweeter and tastier than the standard imported banana (which I believe come from the Philippines). You really can't go wrong with chocolate and cream, though I'm guessing this won't live up to its potential if the banana is fake.
If you find yourself ordering one of these (I wouldn't try this as I love bananas, but not things flavored with banana), let me know what you think.