Remember the warlock is still restricted to cursing only the closest target - just keep a normal monster between the warlock and the minons and you are good.
Also if the warlock is using his curse to take out minons, he is not getting his extra damage on the normal monsters.
This right here is 100% accurate.
I'm the DM of a campaign that has a warlock with a Rod of Reaving. Near the end of the first adventure he acquired it in as treasure. In other words, I actually GAVE it to him. The group just hit 5th level last night, so I've had the chance to watch him play this warlock through about 3+ levels with his Rod of Reaving. And to add to that, this is our resident min-max'er of our group.
Two things several limit him:
1) He can only curse the closest target. He's one of those crazy teleporting fey warlocks though, so he's able to move around pretty well, especially after killing a minion and getting his pact boon. But still, since most warlock powers are single target (at wills anyway), he essentially ends up killing a minion with his minor action and damage someone else with his standard action. The death of one minion per round really isn't that big of a deal in the grand scheme of battle. When he plays that minion slaying role, he's really just clean up for so the other characters don't waste actions on the minions.
2) Every time he drops a minion, that's one real target he didn't get to curse and get the extra d6 of damage towards. I think he's starting to notice the detriment this has on his overall damage, because he appears to be trying to curse the bigger targets first and then do the minion clean up later on.
The other characters (especially AOE PCs like the Wizard) have much better ways of cleaning out minions than a warlock being able to auto-kill one each round. And the Wizard with an extremely high Int (like all wizards do) is coming awfully close to auto-hitting the minions anyway, since they tend to have much weaker defenses than their more robust counterparts.
So I say let it in play and don't worry about it. It really does balance itself.
But...I am not in the camp that thinks that awfully cheap Rod of Reaving/Rod of Corruption gimmick works. That one I would no way allow, nor do I believe the rules allow it either. But that's another whole series of posts...
El Cucuy