The engine takes much longer to warm up on idle. When you run a cold engine the emissions are particularly acidic and collect in the oil.
So when you just idle a car you're not really warming it up and you are building up the acids in the oil and it is likely to be building up carbon deposits much more quickly than it otherwise would.
Best way to warm up an engine is to drive it at the bottom third of it's rev range.
When an engine is already warm idling is not so much of a problem and you'll need to keep it running in traffic jams if you want the aircon to run without draining the battery.
Whilst stuck in snow I idled my engine for 4 hours and it was fine, although it had warmed up, that was a long story.