It takes more fuel to star the engine than to let it idle.
Idling for longer than 10 seconds uses more fuel than restarting.
Idling is necessary to warm up the engine.
On cold days, most vehicles are ready to go in a few seconds and warm up faster by being driven.
Idling is better for the engine than stopping and starting it.
On the contrary; an idling engine can be damaged by fuel residue and water condensation.
Idling will help avoid parking tickets.
An idling vehicle in a no-parking or no-stopping zone is as much in violation as one with the motor off.