+1 on what others have said. When temperatures might vary by 30° or 40° over the course of a ride, it's nice to have a little extra coverage that packs up small when you don't want it. (I'm not above rocking the Flashdance look, either. )

I've got two pairs of PI arm warmers (one wool, one synthetic fleece) that I like reasonably well - they stay up fine and cover my whole arms.

I've also got a pair of Adidas knee warmers that I bought in a pinch on a chilly day, that I really don't like. They're too short in the thigh and too tight in the calf. I once borrowed someone's PI knee warmers that fit me well. I may have to get a pair of those for myself, and throw the Adidas ones into the "free" bin here.