I haven't brewed a lot of batches to this point. Still under 20, and probably opening myself up to huge ridicule, but if you see no airlock activity, do people really just go ahead and let it ride for 2 or 3 weeks? Maybe it's just been dumb luck, but all of my brews have bubbled through the airlock. I ferment in buckets, so I can't tell by quick glance weather or not anything else is going on, but if by day 4, I haven't seen any airlock activity, I would at least give the beer itself a look to make sure something was going on.
Don't get me wrong, I understand the idea behind "Airlock is not an indicator" but, I still feel it a bit of a disservice to tell people never to worry about airlock activity. If you don't see anything, ever, I would say it's time to check something, maybe your lid isn't shut, bad yeast, etc. But I would rather just check on Day 4 and either be at east or fix the problem, than wait two weeks to find out that something unwanted was growing in my beer because my yeast never took off.
Yes, Hydrometer reading should always be the end all be all of weather a beer is done, but to say no activity ever, is always OK, seems wrong to this novice. I think a better way to say it would be, don't let bubbles be an indicator that fermentation has stopped. Like I said, this is only because I have yet to have a situation wher I didn't get bubbles in teh airlock. And with the amount of CO2 being produced, it would seem odd to me that you would never see a bubble, unless something was amiss.