Shooting in the hot sun where your barrel is already to hot to touch before you start is a bad place to require long strings of fire. I don't want to burn one up gratuitously, and I think it's really rude for a MD to put an extended fire stage in a match in the desert. However, since the majority of stages are 10rnd, and my stick-on thermometer never gets anywhere near the danger zone, unless the rifle is sitting in the sun before my turn, for 10 shots then two more shots isn't going to make any difference.
I've shot matches where I carried a wet towel to cover my barrel between stages because it was so hot and nowhere to set the rifle in the shade. How much this preserves the barrel life I don't know, but it makes me feel better about it.
I just don't want a barrel to let go during a match. I've seen it happen, and seen the frustration on the guy's face when all of a sudden his shots started to musket-ball all over the targets. It's usually when they were trying to squeeze one more match and knew it was a high round count. I shoot a switch barrel and keep a spare precisely because I've seen this happen and it sucks.