My husband uses the keycard exclusively since his app is set to his Model S by default and that doesn’t play well with the 3. So the fob gives him added functionality over the card.
I like to leave my 3 unlocked in my garage, so with the fob, I can turn off walk away lock and still easily lock my car as I walk away when out and about (I keep the fob hanging on the outside of my purse)
And, today the fob was quicker than digging out my phone when I walked up to my trunk and the phone key didn’t get recognized, again. Hands full of suitcases coming home from the airport, I just double clicked on the fob after the initial couple of pulls on the trunk didn’t work. I would have had to set stuff down to find my phone in my purse.