I also just noticed that the original poster is assuming that they aren't operating precisely by the standards of the Unearthed Arcana rules for Armor as DR, and are, in theory, simply ruling that whatever armor bonus to AC an armor would normally grant is instead converted to DR X/--.
So, the Breastplate is granting DR 5/--. But, still, just remind the DM that his meleeing monsters usually have a better attack bonus than a PC meleer, and remind him further to use Power Attack. A lot.
For an even more subpar example, let's say you're wearing a Chain Shirt at 1st level and you're a Fighter with Dex 13. You've got an AC of 17 and DR 4/--. Not too bad. Your party faces two Orc Warriors both with Str 20 with Power Attack and wielding Longswords. They run in, wielding those Longswords in both hands, and they swing in Power Attacking for 1 each. Their attacks are at +5 (1 better than your Fighter's even with the penalty from PA) and deal 1d8+7 damage, for an average of 11.5 damage. If they were coming at you with Glaives or Greatswords that's a whole lot more damage (14.5 with the Glaive, 16 even with the Greatsword). Suddenly that Armor as DR doesn't look so scary.
Hell, ALSO remind him that any enemies' natural armor would grant them an equal amount of DR/--.