One seemingly unnecessary item I always take is a full set of medium weight long underwear, including balaclava. They weigh next to nothing and pack small. Even in tropical areas you can be surprised by a cold snap.
One example: In July 2007, I was on a trip in southern and central Brasil, not the sort of place you'd expect to need long underwear. Most of the clothes in my bag were lightweight synthetics, easy to wash and dry. I had a broad-brimmed sun hat, one windbreaker and one light jacket.
An unusual cold front came up from Antarctica, bringing snow as far north as Buenos Aires. Where I was visiting, the temperature in the mornings was 45F. My long underwear really saved me, and allowed me to continue my planned activities. A couple of days later, temperatures returned to normal.
Don't leave home without them!