Taken from a thread on here
It is certainly more technical and proficient descenders do have plenty of opportunity to make time up on the descents on the XC race whippets.
In general I have never seen this to be the case. Generally amongst your average rider at an event your good climber is as good or better than the slower climber. They get fitter by spending time on the bike which improves skills generally.
If you are a good descender then start to get much fitter, do you loose the skill? I doubt it.
The only reason I can think of is bike choice. Someone who is a good climber will pick a bike for an event that compliments there skill for climbing while not necessarily being ideal for a fast downhill run. Still seeing good xc racers, they are not short on skill or bravery when descending.