Why wouldn't you just use the phone? The only way the watch will use cellular is if the phone isn't with you... and why would anyone not use their phone as key? I mean, are we cavemen using RFID wallet cards???
Now that on watch os 6 they have the ability to do stand alone apps, it would be cool if Tesla did a watch app so the watch could be used as a key independently of the phone. Even so, it can't imagine this situation coming up very often, if ever.