People who know to use blacksmithing on their plate armour are happy people.
People who don't know to do this are very sad indeed.
In all seriousness, I don't think that's good design. Moving slower is not the appropriate way to punish players who aren't aware of such things.
I think it's perfectly normal that Plate armor makes you move slower in combat.
I think it's kinda dumb that you can hammer on a breastplate and remove its movement penalty (which also devalues the Armor skill)
