The bag of rats can be used to exploit several loopholes.
The original is a fighter carrying around a BoR with the purpose of throwing them at an enemy, then using Whirlwind Attack to attack them all in one round, and Great Cleave to get a free hit at the actual enemy for each rat killed.
A similar issue occurs in the 4th Edition with the Warlock. Some powers like Misty Step activate upon the death of a cursed enemy. This could be exploited by carrying a bag of rats, cursing them, and killing them at the appropiate times, in order to use Misty Step at will.
The Player's Handbook doesn't specify any way for a Warlock to use powers like Misty Step outside combat, which could also result in using a bag of rats to be able to perform it at any time.