Loc: Portland, OR
In 70 years of backpacking, I've never felt the need of a thermometer.
Colin Fletcher always took one with him. Thermometers are given a short section in The Complete Walker and are mentioned in passing in his other books. He seemed to enjoy running about measuring the temperature of things, such as the interior of his boots after walking in the desert. He never suggests he thought it was a necessary or practical item.
I've taken a thermometer with me on occasion, but in the same spirit as Colin Fletcher. It was entertainment, not critical gear.