A few years ago I read a bunch of Bernard Cornwell's "Sharpe's" series of historical novels set in the wars against Napoleon. In the major battles, there was a sort of "Rock, Paper, Scissors" thing going on. If the infantry formed up into a tight-packed square, it was invulnerable to cavalry charges; but it was then much more vulnerable to cannon fire. If the infantry was spread out in a loose formation, it was pretty safe from cannon fire, but could get wiped out easily by a cavalry charge.
A sort of similar thing has been evidence in our D&D game. If a party or group of monsters spreads out, then they can get swarmed, ganged up and royally bitch-slapped. If the group goes all tight-formation, then they are a lot safer in melee, but much more vulnerable to spell attacks.
