Ilbranteloth
Explorer
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.
Weapon damage rules were part of both the original and 4e Dark Sun settings if I recall.