I guess I had this question inspired by PAA; I’m not sure why I never noticed it before.
Malfoy became the Master of the Elder Wand by disarming Dumbledore with “Expelliarmus”. So why do wands not change allegiance every time anybody used the disarming spell?
Some are easier to answer. Apparently, the spell itself isn’t enough to defeat somebody. Like when Snape killed Dumbledore, it wasn’t called defeating him, because Dumbledore wanted him to [besides for the fact that Malfoy had already disarmed him!] – In Dumbledores Army and the Duelling Club, the duels were set up willingly, and there was no harm intended. Any ‘friendly’ duel could be answered that way. But any time it was used without the ‘consent’ of the subject of the spell, why wouldn’t it win the allegiance?
There could also be a slight difference on the situation the spell is used in. If the spell is simply to disarm, but not in the context of a genuine fight (For example – when Harry uses it to get the diary from Draco Malfoy in Book 2, where he used it to try accomplish something, but not trying to ‘defeat’) then it wouldn’t ‘defeat’. It would have to be a spell that was a gamechanger in a real power struggle.