I think it really depends on what service the transfer provides. For some hotels the transfer is just a booking with an outside limo company and provides you nothing more than a ride. For hotels that actually have their own cars (i.e. not just booking a limo through a different company and charging a markup) I think the service is definitely worth it, espessualy when you consider how much you are spending per night for exceptional service.

As an example, last year my wife and I stayed at FS Florence and FS Paris and booked the airport transfer through the hotel. In Florence, we were greeted at the car by the GM of the hotel as well as the Director of Rooms instantly whisked away by a staff member who gave us a quick tour of the grounds and shown to our room, where we did a private check in.

In Paris we received a call from the GM on the way to the hotel asking if we needed anything on arrival. Once there our upgraded room wasn't yet available so we were taken directly from the car to the courtyard and provided a bottle of Veuve Clicquot Champagne for free. Once our room was available we were fetched by the staff and taken to our room where our bags were waiting for us.

In both cases while the price was much more than a taxi or town-car but the service we received more than made up for the price. (FYI we were booked by a FSPP so that could have played a role as well)