The point that I was trying to make wasn't a point against headsets. It was that they perform their intended purpose well, but you don't see most end users with them. Headset just cutout one piece, holding the device in your hand while doing audio tasks.
Likewise some of the watches are very similar, some units have impressive battery lives and allow you to do GPS/accelerometer or visual tasks that require viewing only.
In the end, your point rings true. What "killer app" is would be more convenient on a tiny screen strapped to you with little input capabilities rather than the multifunctional device you have to will also have to carry in your pocket that has already been done (notifications/fitness)? That's the answer the OP's looking for and I think the collective answer is "I can't think of one".