Re: Please don't do that
I would not have an objection to a bank uniquely identifying my specific phone as a way to require less authentication, so long as the API that did that and permission it would ask for was something Apple had to specifically approve for an app to reach the app store.
Basically it would be sort of two factor authentication without requiring a separate dongle - I have MY phone and MY password, so they know for sure it is me. But the possibility of abuse is so high I'd want it to be something Apple controlled pretty tightly and only allowed apps from approved publishers for a very limited number of apps that deal with financial stuff like banks, brokerages, etc. and the user would have to approve it on their end as well.
Being able to know it is MY phone is too useful to throw away completely just because 99% of the market would abuse it for advertising purposes or worse. So I'm not ready to say "don't allow it at all" but it would need to be treated with extreme care.