I think riding style should determine your helmet choice. How technical are the trails you ride? How fast do you ride? If you're into major rock climbing or the types of riding where you could slam your head into the handlebars, I think a closed-face helmet should be worn... be it an MX, a regular one, or a lift-face one like my HJC.
Now, a caveat about the lift-face (modular) helmets: The latch mechanism is not known to take direct impacts well. That means if there's an impact that comes up from underneath, the latch is likely to fail and cause the face of the helmet to lift, exposing your face. I know this and am aware of it. I feel that I'm more likely to take a direct facial impact and will take the chance. Plus I like the option of using it as an open-face most of the time.
Rob