I agree with you, light switches on their own aren't a compelling case for AR (though side-note: it's the RGB or tunable color temperature adjustment that tends to make IoT lighting worth considering, line voltage switches and bulbs don't do that). It's just one application of many, which I think are eventually going to be enough that it'll gain widespread adoption.
Beyond interactivity with connected objects, the other big opening seems like "contextual information on real-world objects." Some cases for that are industrial (say a factory where a floor manager can see sensor information overlaid on the production line), and will have a much easier time getting adoption because the cost and benefit are more drastic.
Other cases are consumer oriented: better interfaces for mapping/directions, personal tour guides (for museums or cities, and maybe on rental hardware before it hits wide adoption), more immersive AR games like Pokemon Go and Ingress, AR fitness trainers where you can see your personal "best time" ghost running in front of you, video conferencing where instead of putting somebody on a screen they can sort-of exist in the same space with you, etc.
I don't think any one of those is the single application where 90% of people say "I need one of these," but the combined weight of them together is something I'd want.
Heck, you can even keep your smartphone if we need it as an input, networking, and computational device, but wearing AR glasses can effectively make its screen larger so that maps and the like display past the edge of the hardware. Or maybe instead of covering the screen contents, your notification banners pop up above/below the physical device (I don't want literally everything popping up on a HUD, most notifications aren't that important). Or if you have a large scrolling list of open apps, instead of flipping through them on the phone screen, you can just reach out in space and grab the one 5-screens to the left. Even if the pixel density isn't as high as a modern cell phone, having AR on gives you interesting options for extending the device. Maybe it even reigns in the screen size back to 4" phones, and the $200 you can knock off the phone hardware helps cover the AR costs.
I don't claim to know exactly where we're headed, but I don't think there's a shortage of interesting uses. And maybe the runaway feature is something none of us have conceived of yet.