Bearing in mind that most peoples' weapons were personal property back then, would you really throw a (relatively) expensive pistol (even a cheap one would be expensive to the average Highlander) and risk somebody else having a windfall find? I know post-battle brawls were common, but I always imagined they were over women, whiskey, or cards, or other less important things, rather than "Hey that's my pistol in your belt!"
One-on-one life or death tactic of throwing your pistol or tomahawk I can see, but a mass throwing of weapons is bound to end in a punch-up when it comes time to recover them.