Well the obvious advantage a implant has over cards and fobs and such is that you always have it- you can't lose it or leave it at home in your other pants (as I'm apt to do). As for "accidental" purchases- there's ways to prevent that- such as typing in a short PIN number during a transaction. People who use debit cards here do that now.
And we haven't even talked about another important application- access control- a door knob can be programmed to open only for authorized people. The process would be so automatic that the person wouldn't even think about it- he just turns the knob and it opens. But if a unauthorized person turns the knob it won't open. This could be used for all kinds of things- like guns that will fire only for their owner.