Well, there are battles and there are skirmishes. In battles, as other have said, certainly before WWI the smallest maneuver element was still pretty large. In the ancient world, the smallest individual maneuver element in a given army might be an entire legion! Troops advanced and held positions en masse and command and control were not particularly advanced.
In a skirmish situation it might be more difficult to judge. Certainly there are weapons available in D&D such as the pike which would be positively devastating in a battle and laughably useless in a skirmish. You'd probably need one of the ARMA geeks to weigh in on medieval skirmish tactics, as it's a fairly obscure question!
Now, one can also go with the idea that the D&D party is anachronistically related the modern Vietnam-era infantry squad (or half squad, actually, in most games). This casts the Mage as the squad heavy (the guy who would be carrying the M60), the Thief as the point man, the Cleric as the combat medic, etc. But in real terms the analogy breaks down pretty quickly because, also as mentioned above, suppression, fire and maneuver, enfilading fire, etc. more or less lose their meaning in the D&D mechanics. The Mage can be likened to a machinegun, but he really isn't one, etc.