Baseball bats are not and never have been useful on the battelefield. There’s a good reason why nobody, not even stone age cultures, every used weapons shaped like baseball bats. Think about it. A baseball bat is not a difficult shape to make, yet no culture ever used such a weapon. Lengths of straight wood the same length - shortstaffs - were ubiquitous, as were both shorter and longer lengths with straight handles and/or a knob end. I know people have this image of a caveman dragging a big club like an over-sized baseball bat, but such weapons never existed.But never was the baseball bat shape used. That’s because it absolutely sucks as a weapon. In a battlefield situation. I can train any reasonably fit person to disarm an attacker wielding a baseball bat in less than 15 minutes, and the technique is >99% effective. I would rather face somebody with a baseball bat than someone using their bare hands, and I would much, much rather face someone with a bat than someone with a piece of shovel handle the same length. Baseball bats are completely wrong as weapons.
The problem with baseball bats is that they are designed to deliver a lot of force to a predictable. spherical target.
Look at how a baseball batter stands: side on to the target he is hitting. The bat is carefully designed to deliver its power at the very end of its momentum at a point where the users arms are fully extended. That’s a great way to deliver a lot of force, but it’s shit for hitting an unpredictable target. If the target moved half a step inwards, the batter will be striking with his forearms, and won’t do any damage at all. If the attacker moves half a step outwards the attacker will miss altogether because he is already at full stretch with arms, feet and torso. That is the position that baseball bats are carefully designed to be used form, and it’s utterly useless in a fight. Compare the stance of a batter to the stance of someone fighting with a sword, mace or shortstaff. The batter stands side on to his target, the weapon uses face his target. The batter can strike, without changing stance, in a ~ 150o arc from a point at right angles to the attacker to a point at the midsection of his attacker. If his attacker moves even a little to the right he literally can’t be hit without the batter stepping forwards. The weapon fighter also strikes a 150o arc, but that arc is *centred *on the attacker. the attacker can’t just step out of it.
You can try this your self, Hold one hand in the other and see where in from of you you feel you could exert force. Because the arms are locked to the weapon, the strike range is centred directly between the shoulders. But a baseball bat is carefully engineered to strike at or near the extreme end of that strike range. That’s where the bat has the most momentum, so it’s great for hitting a predictable target, but it’s useless for trying to strike a target that see the blow coming and move aside. Such a blow delivers a lot of force, but it’s a literal haymaker. It’s slow, you can see it coming and it’s ridiculously simple to step away from or parry.
So, The only way to effectively use a baseball bat in a fight is to stop using it as a baseball bat and try to use it as a shortstaff or shillaleagh. But it is too thick to be gripped as a shortstaff, the striking surfaces are so thick that they are largely ineffective as a shortstaff and the bat is clearly unbalanced, with all the weight well past the midpoint of the weapon. A piece of wood is much more effective as a shortstaff than a baseball bat, so the bat is useless on the battlefield to the extent that it’s less useful than a length of wood.
So you can try using the bat as a one handed shillaleagh. Except that it is way to heavy to be easily used one handed, and the striking surfaces are so thick that they are largely ineffective if applied with single handed force. So the baseball bat is much less useful than a basic shillaleagh like a wooded mallet or knobkerrie that are light and short enough to use single handed and with a point that concentrates the force of the blow.
A baseball bat is also the wrong shape for a weapon. It has a broad striking surface, which is great for striking a relatively inflexible sphere, where the point of contact is infinitely small. But, aside from the very top of the head, the human body is neither spherical nor inflexible. A useful weapon needs to have a contact point that is narrow. so it concentrates the force. A blade shape is good, as is a sphere. Even a basic narrow cylinder is relatively effective. All those designs have been incorporated into real weapons. But a cylindrical surface as thick as your forearm? That’s just not a practical design.
So to answer the OP, baseball bats were never useful on the battlefield in the sense that anybody would willingly choose one. Soldiers on battlefields have been known to attack each other with rocks and sticks when nothing else was available, so in that situation a baseball bat might be considered “useful”, but no way would anybody select a baseball bat out of the items available at a sporting goods store. Things like tent poles and shovel handles would make a far more effective short staff, and a basic wooden mallet used to drive tent pegs would make a far more effective shillalleagh.
Baseball bats are useless weapons. Used by or against a trained fighter they are worse than nothing and much worse than a straight stick the same length.