well, weapon size can frequently spell the difference between overcoming DR or not, which is why greatswords are "Grrrrreat!" adamantine greatswords are "the shaz-nasty."
I still agree that Monkey Grip, as written, is a stupid feat. however, I've toyed with weapon modification mechanics for years now, and adjusted that feat (it reduces penalties for heavy/awkward/oversized weapons), the mithril property (also reduces oversize penalties) and introduced other things, like Elvencraft (can be used with weapon finesse, -25% weight, +1 threat range) Broad-bladed (+1 AC fighting defensively, +1 slashing damage, -1 attack rolls) Heavy (-2 attack, +1 size catagory to damage dice) Orc-forged (-1 attack, +2 damage) blade-weighted (-1 attack, +1 damage) or Pommel weighted (+1 attack, -1 damage), all of which are technically modifications to the weight and weildiness of a weapon... so monkey grip applies to reduce the penalty.
thats all homebrew shit anyway though, so its kind of invalid for the purposes of the discussion, but lemme tell ya, a raging orc barbarian with max natural strength BEFORE he rages swinging around a heavy, broad-bladed, blade-weighted orc-forged, masterwork mithral falchion is nothing to sneeze at. in fact, its probably nothing to be within charging distance of...