Most competitive shooters don't suffer from screen tearing that much, while games like Diablo 3 or Dota 2 (named a few I recently played) do. At the same time, Diablo 3 doesn't suffer from input lag at all, and I too highly recommend playing it with vsync=on.
Those "few ms" are, in other hand, a huge disadvantage in games like quakelive.
OK, so sure - in a really, REALLY competitive arena, I'd probably suffer through screen tearing to be a few ms ahead of the competition. That's not really the argument, though, right? For 99.9% of people, who are not playing competitively, screen tearing is downright abusive on the eyes.
I just fired up Call of Duty Black Ops 2, with and without vsync, and with vsync+triple buffering.
These are my results:
I'm very sensitive to input lag, as I mentioned. Vsync off = great. Really responsive, no issues. Vsync on = holy crap horrible (with the bearing in mind that most people wouldn't notice. Of course, that's maybe irrelevant since most people are clueless to a/v or input lag anyway). Vsync on + triple buffering = ALMOST (and I do say almost) as good as with vsync off, but without screen tearing.
Honestly, I'd almost be willing to bet that even a "pro" gamer would have a hard time telling the difference between the vsync off and vsync on + triple buffering.
Now, I can't speak for all GAMES, but any games I own (about 400 Steam titles) have never given me an issue in terms of input lag.
But, again, this is subjective. I can't imagine anyone who games on an iMac (after all, we're not serious gamers, right?
And you're absolutely right about Diablo 3. The screen tearing in that with vsync off is just unfreakingbelievable.