I use the locknuts on my road tourer (they only weigh a couple of grams)
because it holds the tube in place when mounting the tire and makes it
easier to get the pump on when the tire is low. I loosen the nut after
mounting to push the stem up over the tire beads, then snug it down to
pump up the tube.

On my ATB I use two on each tube. one sits at the bottom of the stem
inside the rim, the other on the outside. The rim is sandwiched between
the two nuts. This is for when I have to run the tire at real low
pressures (sand, for example) to prevent tube creep. And the inner nut
prevents the rim hole from abrading the base of the valve stem

Having a second nut inside the rim also allows drilling the rim out for
Schraeder (auto) valves. Nice if you find you need a new tube and the
only available source is X-mart.

"May you have the wind at your back.
And a really low gear for the hills!"

Chris Zacho ~ "Your Friendly Neighborhood Wheelman"

Chris'Z Corner
http://www.geocities.com/czcorner