I think for things like road cyclists they should be compulsory as those guys can reach some pretty high speeds, my cousins husband took a corner too fast last weekend and crashed into a wall, think he was doing about 30 and ended up breaking a lot of bones but it could have been a lot worse.
As for other forms of cycling, trials, downhill, cross country, I think it should be down to the rider themselves as they'll know their own limits on a bike and what they're gonna push themselves to do. If your just going down the odd main road to get between trails I dont think it need be compulsory, common sense should be used here.