I also like to carry a thermometer for all of the reasons listed in the other e-mails.

I've used this one for several years: http://www.rei.com/product/745064

I typically hang it outside and it records both the low and the high. It weighs 2 ounces though. I'd prefer a lighter one.

I recently bought an inside/outside high/low thermometer. Sorry, no link. Got it on the internet somewhere. It weighs several ounces but gives me both the high and low of inside the tent and outside the tent. The outside temps are measured by a wire with a sensor on the end.