Meh, Plate Armour tends to be suboptimal anyway - I think in my group we've had 1 character in 3rd edition that's bothered with it.
We tend to have a lot of situations where Dexterity is important, so maybe my perspective's a bit skewed.
And I'm not saying give the heavy armour proficiency for free, all druids get light armour, I only change the weapon profs to thematically appropriate - but if a druid wants to spend two feats to be able to wear full plate then I'm not going to deny them on the premise that they can't wear metal.