I think the only reason the MF on barrels thing was abused and nerfed was because of two reasons:
(1) Inferno was so damn difficult at the time. It was easier to create a game in act 4 break the few "barrels" in certain set locations and then recreate to do it again. If you couldn't kill the monsters this was the next best thing.
(2) Items we consider trash today were still amazing at the time. Finding a 900 DPS blue weapon was actually something of value and the fact that many were stuck in Act 1 Inferno made farming barrels in Act 4 Inferno worthwhile.
Remember that Acts 3 and 4 had different level monsters then the other acts at the time.