Some say for the engine to be in its best state operationally and economically it must be around 90C. Engine parts have gaps between them and those gaps can only be filled up only when at 90C as metal parts thermally expand. Under 90C the parts stay loose with each other and that give more chance to parts' wear. Under 90C also makes the ignition harder thus reduces fuel economy.

I like to run cool engine but the above logic sounds reasonable.