No, I don't support Grossman's more extreme or out there theorizing, such as the Sheepdog model or his fallacious beliefs about video games. Likewise, Marshall's ratio of fire has been thoroughly disproved many times over. Neither of these are relevant to this particular discussion however.
The particular observation of Grossman which I cited here was that soldiers generally find it easier to inflict violence on others from a distance, physically and psychologically. This observation is one of the soundest and best supported to be found in On Killing, and is backed up by extensive other observations of human violent behaviour, ranging from accounts of historical battles to studies of historical fencing systems (A particular passion of mine), to analysis of pre-modern weapons, to the behaviour of modern rioters and tribal violence. Grossman attributed this to an unwillingness or guilt or desire to avoid inflicting violence on other human beings. I think that the observation is sound, but the explanation he gives to it and the conclusions he draws from it are flawed. I think humans do prefer to fight from a physical and psychological distance, not necessarily due to being unwilling to harm others, but due to prioritizing their own safety first and harming their enemies second. Humans like to fight from a distance because it's easier to avoid injury or death yourself in this way. Perhaps a better way of phrasing it would be that humans prefer to fight with the greatest personal safety they can, both physical and psychological senses of safety. A point in favour of this from history is the fact that the real slaughter of battle is always accounted as being in the rout, not in the actual combat against resisting opponents. The sense of safety gained from striking fleeing enemies down without resistance allows for people to strike at will, without fear of receiving injury or death in return.
Hand to hand violence is exhausting. Anyone who has boxed or wrestled can attest to how much it taxes every element of your body and mind. I personally practice fencing with a longsword, and a sparring bout (Which doesn't even have life or death on the line!) is physically exhausting. A competition bout in a tournament, with honour and status on the line, is incredibly mentally fatiguing in addition to physically fatiguing. How much more exhausting is a hand to hand encounter on the battlefield, with a great possibility of death or injury involved?
Hand to hand combat cannot be sustained for very long. Yet we know that battles are accounted as lasting for hours at times, even for most of a day. They physically can't have spent most of their time fighting with the gladius, and I think in most of the gladius charges the Romans made, the enemy would have recoiled backwards or ran before blows were ever exchanged. Thus the pilum would have, in all probability, been the primary weapon of the legionary for the majority of a battle.
The analogy is thus: The musketeer is a man armed primarily with a missile weapon, whose primary task in battle is usually to attack the enemy with said missile weapon. However his fire alone rarely can decisively break an enemy's will and make them flee the battleground. Thus he is also armed with the bayonet, which transforms his musket into a spear, which communicates lethal intent to the enemy, and which employed in a charge by determined troops will break the enemy's morale. The bayonet did not kill great numbers of the enemy, bayonet wounds were vanishingly rare in field battles and were very often a result of using the bayonet to finish off wounded men. However the prospect of facing the bayonet was so terrifying to men that they would rather run from it, thus giving the bayonet charge its moral force and its impact on the battlefield of the musket era. Here I will quote Ardant du Picq:
Battle Studies, Part 2, Chapter 2The shock is a mere term. The de Saxe, the Bugeaud theory: "Close with the bayonet and with fire action at close quarters. That is what kills people and the victor is the one who kills most," is not founded on fact. No enemy awaits you if you are determined, and never, never, never, are two equal determinations opposed to each other. It is well known to everybody, to all nations, that the French have never met any one who resisted a bayonet charge.
The English in Spain, marching resolutely in face of the charges of the French in column, have always defeated them.... The English were not dismayed at the mass. If Napoleon had recalled the defeat of the giants of the Armada by the English vessels, he might not have ordered the use of the d'Erlon column.
Blücher in his instructions to his troops, recalled that the French have never held out before the resolute march of the Prussians in attack column....
Suvaroff used no better tactics. Yet his battalions in Italy drove us at the point of their bayonets.
Each nation in Europe says: "No one stands his ground before a bayonet charge made by us." All are right. The French, no more than others, resist a resolute attack. All are persuaded that their attacks are irresistable; that an advance will frighten the enemy into flight. Whether the bayonet be fixed or in the scabbard makes no difference....
[...]
To sum up: there is no shock of infantry on infantry. There is no physical impulse, no force of mass. There is but a moral impulse.
The Legionary may not have drilled in formations or thrown his pila in volleys like the musketeer did, but he is likewise a man armed primarily with missile weapons. Most of his time spent in battle, I believe, would have been spent on throwing the pila at their opponent, moving forward out of the group to skirmish and then falling back into the perceived safety of the maniple or cohort's mass. But his throwing of pila, like the fire of the musket, cannot put the enemy to flight, it lacks the moral impulse or shock to force men to rout. His gladius, then, is his bayonet. The gladius communicates lethal intent, the gladius is a weapon of terror. The gladius decisively ends the missile exchange, the centurion ordering the drawing of swords stops his troops from continuing to skirmish indecisively (Assuming they listen to him, as evidently they often did not) and commits them to close the distance. Actual exchanges of blows, of gladius against spear, might be rare on the ancient battlefield. Probably more common than bayonet wounds in the musket era, but more rare than injuries or death by spear or thrown missile. Even so, the gladius charge is the shock action, the psychological shock, which decides the combat, just as the bayonet charge decides the musket battle.
And, I would note, when the Roman legionaries find themselves in frontal combat against an infantry formation which is arrayed such that they cannot get at them with the gladius, a formation which keeps the Romans at bay and is not terrified by the gladius charge, the Roman lose. The Roman infantry were losing, slowly but surely, at Cynoscephelae and at Pydna, until the battle was decided elsewhere on the field.
Finally: I have not said that the Romans conquered their enemies by "superior indiscipline". That is a mischaracterization what I have been arguing. My argument is that the Romans won their wars because the Roman culture raised its people to be brave, because Roman society and politics made it easier and preferable to be brave than to not be brave. The Roman army gave scope for its brave men to win renown by great deeds, but kept them under enough control that they were more than just an armed gang. I believe that Romans achieved a hard-fought and tense compromise between individual aggression and group cohesion, which allowed them to both have valour and wisdom, virtus and prudentia both in measure. But I emphasize that this was a tense compromise, and one which disciplina did not always win. They were closer in spirit and ethos to their Gallic enemies than I think they might have liked to think.