The only thing I use airlock activity for is, along with krausen drop, a general guide to when to bother taking a hydrometer reading. I stand by that 100%. As I've said before, if you're using an airlock (I don't use airlocks much anymore, actually) and not getting any bubbles, then something is wrong with your seals (if you're using a bucket). Not that that matters, but if you put a bung into a carboy and put an airlock in that bung, you should see airlock bubbles during fermentation. If not, then either your glass/better bottle has holes in it, or you're living in an alternate universe where yeast doesn't produce co2 as a byproduct of fermentation. I've had airlock bubbles every single time I've ever used an airlock during fermentation...and every time, when the airlock bubbles slow down, and the krausen falls, it's been finished (or stalled, in a few instances). I never said you should use it for any final determination, just as a general guide (along with krausen drop) as to when it's prescient to take a hydrometer reading.
Of course, if you're like Revvy and for some odd reason have holes in your carboys, or you're like me, and use aluminum foil in place of a bung/airlock, then just pay attention to the krausen.