My concern with the proximity key system is that it's a pretty complex system, and seems to me like it could be failure-prone after a few years. A key always works. Whoever mentioned breaking the key off in the ignition...have you ever done that?! Never even heard of people doing that.
As just one example, Car and Driver had a long-term 2006 330i, and the electronic key-fob slot failed within the first year. The solution? Flatbed it to a dealer. I've never, ever heard of a traditional key ignition failing within the first year on any car.