I don't know if you have ever fought against dwarven heroes at level 10 with Mithril, but its a nightmare. Especially with the Ered Luin faction and Balin's ability to give those runes to other heroes. Now think about adding that incredible endurance to armies of elite troops, and you can see why this would be pretty OP. Also, late game money isn't really an issue, by then a good player is probably floating like 20k resources, so he could afford to have a whole army of elite troops equipped with Mithril Mail. Even now, a fully upgraded army of Uruk Hai with shieldbearers attached to them is almost unbeatable, giving dwarven units Mithril Mail would make fully upgraded Uruk Hai look like pathetic orcs in comparison. The Mithril Mail should stay hero exclusive, or if put on units it would have to be severely weakened to keep the game balanced, which would defeat the purpose of it being Mithril.