It's strange to see in action, and there will only be so many people who have both Android and iOS devices and want to use one to sign in to the other. Google also warns that it would be wise to have a backup physical security key in case you ever lose your phone. This could be particularly useful if you're in a corporate environment where you need multiple devices, though, as you won't have to worry so much about hackers compromising vital data. It could also help you mix up platforms at home without worrying that security will fall by the wayside.