As I noted above, a Pit Fiend w/weenie minions is a form of BoR tactics... The DMG may rail against BoRs, but the MM appears to be condoning them. And that doesn't even begin to get into the complexity of defining a BoR when the BoR's contents are creatures that *are* legitimate targets, elsewhere.
For example, Sunless Citadel *has* an encounter with normal rats. I do hope you wouldn't prevent an fighter w/cleave from using that feat in that encounter; but if you let him, then clearly rats are a possible cleave source...