My son wants to get metal cleats and I suppose that I will allow him to do so. I have expressed my concerns and I have shared with him the risks. Like most decisions, I will allow him to make his own educated decisions.
Injury Risk Number One - Cleating yourself - and perhaps one of the most annoying things that I remember from wearing metal baseball cleats when I was young - cleating yourself. I remember the pain of the metal cleat hitting my own ankle. This is more of an annoyance than anything.
Injury Risk Number Two - Getting Cleated by your opponent - One of the most common "metal cleat injuries" occurs when a base runner slides into a base and spikes the fielder. As the runner slides the cleats are exposed toward the fielder's glove, leg or foot. Of course a runner sliding head first can get his hand stepped on by a fielder with metal cleats.
Injury Risk Number Three - Too much traction - Players prefer metal cleats because they provide superior traction, however too much traction at the wrong time can result in a serious knee or ankle injury. In the game of baseball quick changes of direction happen all the time.
(Example One) Aggressive players always look to turn a single into a double by hustling down to first and taking an aggressive turn after a hit to the outfield. If the outfielder bobbles the ball they are ready to take advantage, but if the player fields the ball clean and comes up throwing, the runner quickly reverses direction. If the runner's metal cleats dig in deep as the player is turning, a player can damage knee or ankle ligaments. This is exactly how I blow out my ACL playing men's fast-pitch softball. And I was wearing plastic molded cleats.
(Example Two) I have also seen catchers hurt their knees blocking the plate. As the catcher gets the ball, he is suppose to position his left foot on the baseline facing third base, but many put their left foot perpendicular to the line to block more of the plate. A foot anchored in metal cleats at an improper angle could increase the likelihood of knee and ankle injuries during a play at the plate. (See Baseball-Catcher.com for proper positioning advice)