visors become a problem when you are leaned way over. you need to look up in order to avoid major neck angles and the visor is right there. (some sunglass frames are bad too, and should be considered when you go shopping for eyewear.)

leaning way over happens frequently on road bikes, thus many road helmets don't come with visors. on a MTB it happens a lot less frequently, for many people never, and thus visors are common.