Zardnaar
Legend
Let's turn that around. If a metal sword is as good as a +1, what's important about having +1 metal swords?
Seriously, though, weapons can differ in ways beyond bonuses or penalties. A simple and easy to use weapon breakage rule, for example, is quite genre-appropriate.
Likewise, changing the damage die would be a quick way to differentiate them.
An actual penalty makes sense. The Aztecs for example had trouble vs Spanish steel. Another way of doing it I suppose would be to have inferior Athasian weapons that are simple weapons or a 1d10 greatsword like the Aztec obsidian sword. If you want a 2d6 one go find a metal one.
I like the penalties because it actually makes weapons inferior and there is a trade off (and incidentally indirectly nerfs the -5/+10 feats). You can suck up the penalty or just use a staff or club which fits the setting instead of everyone using normal weapons.