The problem with knees is that knees don't have many feelings and rarely feel cold, yet the meniscus and other internals don't work properly when it's cold. Many knee doctors recommend covering the knees below 65-70F. In fact, I've read a couple times about national team members and pros being fined for not wearing tights during training if temps warrant it. In races, heavy creams and oils are used to protect the knees but training is a whole other thing.

Btw, I've seen pics of pro teams training on open roads wearing tights while hangers-on wannabes are following in shorts.