Well, I'm in the US where 13.6% of drivers completely suck and screw up driving for the rest of us. That includes people who somehow seem surprised that the light turned green, and sit there completely unprepared for what to do next. Meanwhile, 4 cars that could have followed through are stuck waiting on the next.
So I welcome our green-light warning overlords.
Most of the time it is just someone who got distracted, which isn't particularly dangerous when sitting at a light. I don't think a countdown will help with a lot of cases, not to mention most people only hesitate for a second or less before people are honking at them.
I never leave the second the light turns green as I see at least one car a month blow the red so late I would have been nailed if I started moving the second the light went green. I'd rather be a half a second late through the light, rather than have a totalled car.