Add me, too.Add me to the list of brewers who do look to airlock (or blowoff tube) activity as a sign of active fermentation.
No airlock activity in the hours/days after pitching is indicative of one of several things:
Ineffective fermenter seal (likely no big deal - but a cause nonetheless)
Long lag time
No active yeast
Couple that information with a visual inspection and/or hydrometer reading, and the lack of activity tells you something - either the fermenter is poorly sealed, or the yeast hasn't started working very hard yet (maybe never will). Is it cause for worry? Probably not. Does it provide information? YES! If nothing else, it is an indicator that you might take a hydrometer reading much sooner than you would otherwise.
On the other hand, a very active airlock is indicative of healthy fermentation. It's reassuring. It also provides information. It's not complete information, but it is indeed information.
A slowly bubbling airlock is less useful but not useless. As mentioned previously, atmospheric changes can cause that just as easily as fermentation. However, if the airlock was bubbling rapidly for a period of time, and activity slows/stops, it indicates that fermentation may be coming to a halt. For beers that I want to keg quickly, I use that as an indicator to take a reading. A hydrometer is your friend.
I'm not advocating using airlock activity as the sole means by which you measure the health of your yeast. However, there is information to be gleaned by watching it.