trysha

On Wands;

*) Wands are not exceptionally expensive - 7 galleons.
*) You can use any wand, if you don't have yours available.
*) You can still use a wand if you are holding another wand.

On the disarming spell; Expelliarmus.

This is the standard disarming spell. One can infer the following properties
about the spell from the books:

*) It seems to operate by exerting a large amount of force against an object.
*) It "flings" the object in a certain direction without damaging the object in question.
*) It can fling any object, not just wands.
*) It can be used on multiple objects at the same time.
*) It can be used against multiple targets simultaneously.
*) The more emotional the caster, the stronger the force of the fling, it can fling more than the intended target.
*) Unlike the "assio" charm, only objects in sight that the caster knows are shown as being flung.

SO, WHY THE HELL DOES EVERYONE ONLY HAVE ONE WAND?

If most of my powers were tied to having this one thing, I'd be certain to own at least two. I know how I am with my keys, I am SURE that I'd lose it or misplace it. If the death eaters were out there killing folks randomly, i'd carry at least two on me at all times.

I'd have headed back down to old 'Olly's and purchased the best wand for me that is suited to each school of magic, one for charms, one for transfiguration, one for jinxes, etc.

Heck if i were an auror, a member of the DA, or the Order of the Phoenix, I'd probably carry one "up my sleeve" or somewhere, not even Mad Eye Moody does anything like this, I would have some sort of retractable (non-magical) spring loaded string or chain attaching the end of the wand to an arm band - if it goes flying, it'll come right back on it's own like a latch key.

( sources - some spoilers )