Leaving you engine at idle can result in poor fuel economy, increased emissions and engine damage. In modern engines there should be no need to leave the engine idling to warm it up. It is normal recommendation to start the engine and drive it within 30 seconds. In the days of petrol carburettors there was a tendency to leave the engine running so as to reach its normal working temperature but this could result in unburnt fuel washing the cylinders of oil and as such increasing the wear.

Modern fuel injection systems adjust the fuel according to the temperature/engine requirement. Note that leaving the engine idling does have an effect on the emissions level and should be avoided if possible.