It only strikes me as obvious that destroyed cubes don't spawn new threats, so what is it that makes it so desirable not to take them down one by one (say, RML)?
Destroyed cubes spawn no new threats, but the remaining cubes spawn greater threats: When one cube is destroyed, Raptors spawn instead of BoPs. When TWO cubes are destroyed, Neggys spawn instead of either. On top of that, when a cube goes down in flames, a special group spawns at all surviving cubes.
So, a lower-DPS group may find itself overwhelmed by the increasing strength of spawns, particularly if the cube blows and triggers a special spawn on top of the regulars. Groups with truly excreble DPS may not even be able to withstand mass-raptors, and may be best served by blowing all cubes simultaneously and running like hell for the Kang before things go totally to hell.