[QUOTE=digibud;16057131 but if you let your tire get a bit low on air and you have occasion to brake crazy hard you could conceivably cause the tire so rotate and damage the tube by putting force on the valve. Having the valve screwed down diminishes the chance that the valve will come under a strain like that. It simply holds it in place. Lots of people ignore their use.[/QUOTE]

Having the valve screwed down will rip the valve from the tube if the tire and tube rotate on the rim. We never use nuts on dirt bikes just for this reason. Having this happen on a bicycle will happen once in 10 lifetimes.