I use them when it is cold, but also they work nice when it is cool and the bugs get to knocking on your knees.
Just returned from Langlitz Leathers in Portland, OR. Picked up a pair of their goatskin riding pants (westerns). The go on right over your jeans, they have a 30" zipper so you can put them on with your boots on too. The keep all your lower body warm, the chaps have a cold spot at the top.
Also, when you crash your rear won't get wore down by the asphalt, like it would with chaps. Langlitz has some pictures in their shop to prove it.
Stay safe.