I pretty much always wear one. Even though I might have a relaxed day just staying on the slopes I'll still keep it on. The reason for this is that I was once hit by a board coming down the hill, someone accidentally dropped it while trying to put it on. These things go fast and can kill, so this is also why always wear my backprotector.
Don't think it should be manditory tho, anyone can decide for themselves howmuch risk they feel like taking (maybe not kids, but its up to their parents)
Also, I have been glad at times wearing it, bashing my head into trees and stuff like that going b