It's a question of personal safety. Are helmets, leather jackets, and heavy boots required to ride a motorcycle?
You could be lucky for years riding snowboards and never need the protection that a helmet and wrist guards provide, but the first time that you need then and don't have them, you could be very sorry.
Shaun White took a nasty face plant on the lip of a half pipe last week during the Winter X Games that broke the chin strap of his helmet. Without that helmet, there is a good chance that he would have been seriously hurt.
Conversely, there is a school of thought that believes protective gear is actually causing the snowboarders to take greater risks. "Hey, I just did a single twisting back flip! Now I'm gonna put on my helmet and try a double twisting back flip!" Some boarders put themselves at greater risk of injury believing that the protective gear will save them.
You might just be "cruising the mountain and not doing any crazy tricks", but there are also a lot of other skiers and boarders that are NOT taking it easy. Some of the nastiest injuries I have seen involve two people. Most of the time it is one inexperienced guy colliding with a more experienced guy. Who caused the collision is usually immaterial when both of them are being taken off the mountain with life threatening injuries.
For the most part, protective gear is a great idea. Helmets, wrist guards, and other such protective gear will be beneficial in preventing minor crashes causing major damage.
my two cents