To be honest, the question of whether it's good or bad to leave a car idling for an extended time has never crossed my mind. This letter from Deb made me think about it:
As I read the comments about warming up your car, I keep looking to see if it is actually bad for your engine to leave your car idling too long. I've been told that recently, and just wondered if it was true. Thanks for any help and advice you can give! Deb
So, is it bad? I say no. An idling engine that is properly tuned is an efficient machine. If your engine is at operating temperature, your fuel injection is metering fuel correctly, and your exhaust pipe doesn't have a banana in it, there's nothing to be harmed by letting it run. This begs the question of why you're letting your car idle for a long time. It's bad for the environment to pump all those extra hydrocarbons into the atmosphere and you're wasting precious, not to mention expensive gas. I don't think you'll do your engine any harm, but why do it? Give nature a break and turn it off.