I've not been able to use it during a significant amount of times when arriving at airports that had already gone live with MPC. Some of that may due to my weak cell phone signal or due to airport wifi network issues, but it's also been due to other reasons from time to time.

And at airports where I've been called to the front on stated condition of having the card, I've heard no such call for MPC to come to the front too. Even when a call is not required to get through the herd, finding MPC signage in the arrivals facility isn't always so quick and easy to see.