I had quite a few coin-cell lights, but they’d barely throw enough light to see the keyhole if kept on a keychain. I loved the idea, and had quite a few (shudder to think how many…), but implementation left something to be desired.
Today, more efficient LEDs might make up for it, but back then… nah.
Also, getting squished would so easily turn them on and run them down.
So unless it’s a deeply recessed button to reduce the chances of accidental activation (and even that’s no guarantee), I wouldn’t bother.
If I needed a smaller-the-better light, an AAA twisty would then be best for me.
Alas, I’m EDCing a UT20, a monster of an 18650 light. Doesn’t bug me. So AA, AAA, let alone coin-cell, isn’t that much of an issue.
I got two thingies that slip onto key-blanks (from the factory, non-serviceable) with a red and amber LED built-in. Uh, one each, that is. The red one’s dead at this point, and the amber barely glows. Then again, they’re quite ancient. Those are nifty. Squeeze the key and the LED’s aimed right at the keyhole!
Had one, prehistoric era, with a bigger cell (PX-something? camera battery…) with an actual hotwire bulb! Cell’s replaceable, actually. B’harni knows what I ever did with those (1 pristine, 1 quite “worn”). Separate units, would slide onto an existing key.
Those I’d see a market for, and would get an assload to gift to people, as they’re quite useful.