You missed the point. The wind speed is needed to calculate the power. When paired with another power meter, it can use the wind speed to calculate your cda. It can't calculate the power and the cda at the same time. The whole calibration procedure is to calculate the cda, that's why you do a coast down, so the power is known (to be zero).
You are right; I just realized it will only give you a point in time reading of your drag coefficient when you are coasting. But this would still come in handy to see how different positions / gear affect your CdA.
Find real people who have used them long term and get back to me.
I plan to do that myself; I will be sure to post a detailed review complete with data files. But not having used a PT or similar before, I guess I won't know any better...