My source is my father, who flew 23 different types of aircraft, single engine, multi engine, piston and jet. He spent some years as a flight instructor, and a crash investigator for the Air Force.
He said not properly warming up the piston engine meant that there was cold oil still in the reservoir, and sucking that into the engine would cause it to stall, usually right after takeoff, causing many accidents.
I've read about it in the paper myself a couple times, the report goes like the engine stalled right after takeoff and the airplane crashed. Didn't warm the engine up.
I can see this in my car. If the engine isn't warmed up, pulling into traffic often meant it would stall right in the middle of the lane I'm trying to cross, leaving me vulnerable to being rammed.
Now, if you're in an airplane overloaded with gas and bombs (and combat missions are always overloading the airplanes), you're going to make darn sure the engines are all at normal operating temperature before you start the roll. And you're going to fly right through the smoke of the previous guy whose engine hiccuped after he was committed.
I've flown in a Cessna a few times, and the pilot will always trundle out to the runway, set the brakes, and run it at full power making sure it's warmed up and both magnetos are working. I'd get out if he didn't :-)