since the beginning of mountain bike racing, one of the major differences between it and it's asphalt based cousins has been the concept of self-reliance, no technical or mechanical assistance on the course... if you have a flat and don't have a tube, or pump, or tire levers, you're out of luck. it started in the norba rules and was adopted by the UCI for international competition and it's helped create better and more reliable bikes and components... now, the UCI has decided that "We can't accept that the best rider in the world with a lot of sponsors can lose a race just because of a puncture."
allowing racers to use pits, canibalize teamates bikes, or accept hand-ups of parts is contrary to the spirit of mountain biking, and gives another advantage to racers on highly financed teams who have dedicated mechanics and can now use less reliable lightweight parts with less worry of mechanical failures. sure there are supported races out there, mostly long distance races like the SM100, but there everyone uses neutral support provided by the organizer.
will this affect me? not at all, but i still care... Andrew Juskaitis does a much better job talking about this on the VeloNews website.