I don't think they're even that far off given Bluetooth connectivity and AirPlay. There are some OS things to tweak: I've definitely noticed UI issues using a keyboard with an iPad, for example. But then again, the size of a laptop is no longer much larger than the IO devices: screen, keyboard, and trackpad. Advantages of having a single device is likely data and configuration, or possibly separation of domain, such as work and home. If this is largely sync'd, how much use will there be for these separate devices other than form factor?
Agree, lighting ports on iOS devices also offer a lot of flexibility. Apple's processors also offer far more power than is needed to just run a phone. They seem to be converging towards this.
