Dausuul said:
in a world where fireball-slingers are common, close formation is suicidal.
YMMV, perhaps depending on rules-set, but ...
Unless the formation is also
deep, as in the multiple ranks typical with a pike phalanx, it tends (at least in old D&D) to be rather a waste of fireballs in my opinion. I would rather target that limited resource at high-level fighters and monsters. The heroes and superheroes and lords, as well as their retainers, can cut through masses of normal men like hot knives through butter, putting the survivors to panicked flight -- and do that from sunup to sundown. For that matter, ordinary troops are capable of dealing with their ilk (but not so much of taking on trolls and giants and other fantastic figures, much less those immune to non-magical weapons).
In 1E AD&D, it just becomes more and more "overkill" even versus deep formations of mundane men at arms as the caster rises in level. The number of 3rd-level spells a magic-user can prepare hits 5 at 13th level and does not increase again until 26th.
In any case, I would say that more common impediments would be bows, catapults, cannon, crossbows and handguns -- none of which did away with pikes until the introduction of the socket bayonet spelled the replacement of "pike and shot" formations with shallow lines maximizing musket firepower (by retaining tight files).
Also, many polearms (the halberd, for instance) would be used in rather loose formations, as indicated in the 1E PHB.