Originally Posted by
NichG
Putting an explicit mechanical penalty on something like that makes it a subject of optimization - that is, the druid in character would always want metal armor if they could get away with it, but because there's a cost they may decide to pay the cost to have metal armor, or avoid the cost by avoiding the armor. That establishes a different kind of world than one where 'druids have a mentality that rejects the use of metal armor as a desirable thing'. In the former, druids are mercenaries who only do the things they do in order to trick nature into giving them powers, while in the latter they're true believers in some ideological elements and, as part of their practice, learn to gain powers.