It is next to none when wearing a visor. Now let me ask you this, would you like to be able to use that peripheral vision, which is eliminated when wearing a visor, to get out of the way of a 220lb behemoth traveling at over 40 km/h coming to hit you? I know I would. The point that I am trying to get at here is that these men can make decisions on their own. They have been doing it for several years so why would you change that now.
Forcing players to add visors is just going to add to the already increasing negative aspect of hockey. The well know television commentator, and former Boston Bruins head coach, Don Cherry is a strong believer in this statement. He states "players wear visors as an excuse to use their sticks in a careless fashion. If every player didn't wear a visor then there wouldn't be as many high-sticking penalties or injuries." To tell the players that they have to wear visors is going to give them the impression that they can use their sticks even more carelessly than they are now. It tells them that they are "protected" and that they can just raise their stick as high as they want. This is a false presumption because these visors are not as effective as they are made out to be, which leads me to my next reason in which they should not be mandated.
Although most visors are made to withstand a blow traveling up to 90 miles an hour they are not as effective as most people would think. Being a visor, it only covers half of the players face, leaving the bottom half exposed to anything that might fly its way. Not only is the bottom half exposed but if a stick or puck were to fly upward towards the player's face the visor would be useless as there is nothing stopping the object from going up and under the visor. It is only rare that a blow would come downward as the puck and sticks are all on the ice to begin with. Even if wearing visors was mandated, would the players even wear them properly? If you look at the OHL, a league in which you must wear a visor, the players tip their helmet backward so the visor is out of their line of vision.