Start up issues

I've noticed something lately, and I'm not sure why. Is it a habit that's been around for years and I've simply missed it? And is it actually worth the effort, I wonder. But to the point: four times in the last month I've noticed drivers cutting their engines while stopped at traffic lights, then turning them on when the signal hits green.

Does that save that much gas? I can't imagine that it does. I would also have to believe that it leads to greater wear and tear on the engine. A driver might theoretically stop and restart his car dozens more times a day through the practice. Further, I've long understood (although I concede that I may be wrong on this) that engines use the most gas at start up. Is this true, has it ever been true, or am I simply mistaken? And why would you want to take the admittedly unlikely chance that your vehicle wouldn't restart?

I dunno, but it strikes me as a waste. Its certainly innocuous anyway you think about it. Still, I have to ask, is it really worthwhile?