I can only comment on road racing, but in a word you need both ! You need your distance legs that you get through your base training, but you deffo need your anaerobic stuff (red zone) in place. To quote Jens Voigt recently who said something on the line of
"you dont get dropped from a race because you dont put out a steady constant 280 watts for 3 hrs, you do get dropped from a road race if you cant put out 450 Watts for 5 seconds !!"
Obviously the whole peleton issue makes road racing and mountain bike races different but I guess some of the principles are the same.
Its very difficult to do a race without succesfully having your base and your top-end in good shape. But if I had to line up at the beginning of a road race with only one, I'd trade off my base everytime and try to survive on my top end, because you are dead meat without it. (Unless your races are over 3hrs)