Engine idling is, in fact, bad for engine longevity. More specifically, it punishes the oil.
I had a used engine oil sample analyzed by a laboratory, and the flashpoint was significantly lower by idling the engine warm than the other engine oil samples that were taken after a drive to warm up the engine oil.
Long idling results in increased fuel content in the engine oil, which means the engine oil mixture becomes easier to ignite.
However, my take on warming up the engine is that I find it more beneficial to warm it up by idling.
Spirited drivers like me who are VERY eager to see high load, high RPM use will find long-term benefit of idling an engine warm (versus driving the vehicle to warm up the engine), as the engine will be closer to normal operating temperatures, meaning the engine will be more likely to be at the correct tolerances intended to be operated in high load and/or high RPM use. So despite the loss of fuel efficiency and having engine oil that will likely have decreased lifespan, I consider this a good trade-off for improved engine protection under operating conditions I expect to put the engine through while actually driving.
So when it comes to efficiency, driving to warm up is best, but at the cost of avoiding high load and/or high RPM use until the engine sees normal operating temperatures. But for spirited driving, high load use, and/or high RPM use, idling an engine warm does have its benefits.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano