Train them and use them. As John said, using elbows to the head dangerous in active sparring. You can simulate them if you put your opponent into an especially compromised position to receive an elbow cu de grace and create some distance with a slowed technique.
I have known a person to receive a concussion on the receiving side of an elbow and have seen busted teeth and chops with back elbows. Ain't pretty.
Knees, you may have to limit some of the contact points, but try to use the top of the quads instead of the top of the knee in sparring if using a round knee strike, or when bringing an opponent's head down to a knee---softly.
For blocks, can't imagine not using a rising elbow block to stop a hook to the head.