I have thought about what a good bike computer has that a smartphone doesn't have. Smartphones already have perfectly good GPS, many are waterproof. The list that I came up with is
- ANT+ support
- Battery life
- Button-based interface
- Barometric altimeter
Battery life and button-based interface could both be addressed through a specialized case (and a supporting app)—there are already battery-extender cases and cases with special shutter-release buttons for the camera. ANT+ support would be more of a challenge. It might be possible to build an ANT+ to Bluetooth gateway into a case, although these days, you could just about get away with Bluetooth Low Energy devices. An altimeter, likewise, seems like a "nice to have," not a "must have."
There's also the reassurance that you're not putting all your technological eggs in one basket, I guess.