What if you don't live in Ann Arbor, or what if you live in Ann Arbor, but are not going to Detroit. Or if you are going that way but, the point you are going to, is still 5, 10 miles away from the station. You'd still have to catch a cab or something. Seems like a massive, expensive project that will only benefit a few people.
I'd bet you'd only get a handful of people that would actually ride it everyday, I'm sure alot of people will say, its a great idea, but they won't really use it. It doesn't get you where you really need to go, only in that general direction, unless you work right next to the station, you'll still need to get from the station to your job, And people like the freedom of driving there own car, they may want to stop and eat, go to the doctor's, go shopping, hit the strip club, ect... after work.