Why stop there? Let's attach my SSN to my phone too that way I have every possible personal piece of info available to Google. I can imagine a future where we'll be standing in line awaiting for that slow, old lady to slide her debit card. Seriously granny, I could've been out of here 10 seconds sooner if it weren't for you.
Seriously, it doesn't take any less time to pull a phone out of your pocket than it does a debit card. If you're actually on your phone while in line, you're obnoxious. Everyone else in line hates you. The cashier especially hates you since they need to communicate with you during the transaction and you make that difficult by loudly telling your friend about the cute thing your dog did this morning. What, you think it's safer to leave your card at home? Great, I'll take your phone instead. Unlike the piece of plastic that constitutes your card, the phone has a real value on top of the wealth of valuable info contained within. Your billing address is used as a security check? Hope it isn't the same as your home address, which is probably listed under google maps, which is easily accessible on the phone I just stole from you.