Cycle Racing considered to as a sport but its classification is very detailedness. If someone wear a full face helmet during Time Trial Racing, the wind resistance could not have any advantage to get good performance; while in Downhill Racing who will care for wind resistance since the most important is that our brain should be well protected by the helmets!
Mountain helmets with visor could keep off a splash of debris, but it will affect the line of slight. Therefore it is very important to choose the relevant helmet according to our types of Cycle Racing. Many riders may take several sports at the same time, we should buy different types of helmets if economic is possible; but if we are not well-off or in limited satiation, to choose a universal helmet will be also a good choice, such as for Road and XC (Cross Country), the rider can choose Road helmets, for All Mountain and Street, MTB Half helmet should be enough. But there are exceptional for Downhill and Freeride, since they absolutely require Full face helmet without any replacement.