In my opinion in earlier editions it was purely a mechanism to give the classes some differentiation, like how clerics would only use bludgeoning weapons. I don't mind it, with steel or other worked metal being representative of the 'industrial world' that druids abhor. A bit like Saruman vs Radagast (yes, I know they're both technically wizards, but if you listen to WotC talk about druids they basically stem from the same mythology as wizards).
