First of all, this answer is related, but not equal to, Was Harry making a big mistake with his plan for Elder Wand?.
In the final acts of the HP story, we learn that wand ownership plays a great part. If a wizard uses a wand that does not belong to him, his power is greatly diminished.
Now, a wand belongs to a certain wizard and will change allegiances if the wizard is killed or disarmed by another (for example, Draco disarming Dumbledore). However, we also find out that a wand will change allegiances even when it was not directly involved in the fight that causes it to shift. For example, when Harry disarmed Draco, Draco did not have the elder wand in hand. Still, because Draco was the owner of the elder wand, and Harry defeated him, Harry became the new owner.
Now, this leads me to believe that the rule is
If a wizard is disarmed by another wizard, the second wizard becomes the owner of the first wizard's wands.
This, however, is horrible. This rule means that whenever I disarm someone, I am now the owner of his wand, and there is no way for him to get his wand back without me losing my wand. My wand and his are now locked to have the same owner for eternity. Even worse, if I'm a defence against the dark arts teacher and I am teaching a class to perform the Expelliamus spell, there are bound to be some succesful spells cast, and therefore half of my students will lose ownership of their wands. Probably, after a couple of fights, there will be a handful of students who own 10 or more wands each!
Am I missing some rule here? How is it possible in the HP universe that anyone still has ownership of their wand?