I always wear a helmet on my bikes, but when I'm with cars being highly visible is at least as important. My commuting bike is visible from low earth orbit.
Being visible isn't always a great match to mountain bike racing though. It takes the strongest of minds to race with wheel reflectors or front/rear reflectors. That's not a problem if you never ride your mountain bike on the street in dim light, but most of us at least ride to some trails.
One partial fix to reflector-free bikes is to wear highly visible and reflective clothing - and especially to stick a portable blinkie to your helmet or jacket.
That's good, but you can do even better with 3M based "Lightweights" Power Reflectors. I've put these on several bikes, replacing spoke reflectors and improving visibility.
They are quirky to apply to spokes. I'll try to do a short video; once you know the technique it's easy to do while watching TV or sitting outside with a beverage. Get one end wrapped about spoke and then use the paper end to turn the strip, the paper just peels off. Be sure to clean first with alcohol solution.
If you really want to be seen you can apply some of the 'extras' to bike surfaces. They stay on well. I apply those to cranks (front back outside) and maybe front forks.
The result is very visible at night, and not obvious during day. Total weight added is negligible.