I use it when available and working correctly. At one property I frequent that means I can enter the building from the side door (next to the parking lot) and proceed to my room without stopping by the front desk.

If you have your key card already in hand, it is faster than using the phone. If your phone is already in your hand, can be unlocked with a fingerprint reader and you can get to the mobile key screen quickly via shortcut on home screen it avoids digging for a key card. I also have tendency to demagnetize key cards so having the phone avoids a trip back down the elevator.