This answers the question regarding how much extra fuel is needed to start the engine as opposed to just letting it idle. tl;dw? The amount of fuel required to start the engine was equal to the amount of fuel required to idle for approximately 7 seconds. So if you're sat for any longer than that, then shutting down the engine should save fuel. As for long term wear and tear...well, it's a concern, but I doubt very much it's not one that's been factored in by engineers. Also consider that there are some stop/start systems which don't require the starter motor (so no extra wear on that, or the battery), and these systems also don't activate until the engine is up to temperature, so there's no worries about increased wear due to repeated cold starts. EDIT: and as Fox pointed out, there's also the benefit that it's more pleasant to have the engine shut off when sat for long periods; not just for the driver, but in urban areas, for pedestrians too.