You said the big flaw that keeps smartphones out of the conversation for me: Battery. Aside from that, they are also rather large in comparison to something like an Edge 500, even the relatively small iPhone. Plus there is the fragility issue, and the fact that a touchscreen is not an ideal situation on a bike where you really should be paying attention to the road. There are mounts that provide solutions to many of these complaints, but then you make an already big device even bigger.

Now, if you do not want to see the info as it happens, then a phone tuck safely in your jersey or a saddle bag can get the job done. The battery should last quite some time with the screen off. Then you defeat part of the reason for having a computer in the first place, instant info.