@Gauss Guzzler Does any phone NFC app function while the phone is off? I guess I never really tried, and I know nothing about a phone's NFC hardware, but I always assumed any phone app using NFC would require the phone to be on!Wait. NFC requires the phone to be powered on and functional? Doesn't that entirely eliminate all possible usefulness of the NFC option? If your phone was working, the car would have unlocked before you reached it. Why scrape your phone all over it? Wouldn't it just re-lock and frustrate you?
Phone as NFC key isn't substitute for carrying a separate backup key (NFC or fob). Phone as NFC is still handy for:
1) Backup if Bluetooth is acting up for whatever reason - more convenient than pulling the backup card from my wallet.
2) If I'm getting out of the car, or next to it anyways, then using my phone to tap the driver's B pillar is quicker for me than locking/unlocking via app UI, and also quicker than pulling out the NFC card from my wallet.
No the car doesn't immediately re-lock or re-unlock on you. The car reacts to phone as NFC key exactly the same as NFC keycard. No behavior difference that I've seen.
I use NFC lock via phone all the time now because the Bluetooth connection from my new phone is way too strong, the car stays unlocked until I'm halfway down the block at least. It's actually rather annoying that the car isn't able to better judge my phone's distance.