The question is a bit more complex than it seems. The triarii were the most experienced of the soldiers in the pre-Marian maniples. While the other two types of soldiers used the gladius and pila, the triarii were equipped with spears. It stands to reason, then, that for most of their military career, they used swords.
This leads to a few questions:
Why did they decide to change the weaponry, if they men were trained with swords?
Why did they choose spears? Is spear combat more difficult than using a sword?
What happened when they faced cavalry? Did the triarii take the front lines?
I’d speculate that one of the main advantages would be to allow them the occasional cheapshot from the third line. Poking a spear out could provide a nasty surprise for the opponent who was busy trying to take down the guy in front.
Plus, if there was an actual break in the line, say due to alot of forward momentum during an enemy charge, the triarii might set their spears and probably absorb most or all of it.
And then lastly, there’s the cavalry thing.
Personally, I’ve always been rather suspicious of the notion that the triarii carried only spears, even during the Republican Era. Since they provided their own equipment, I’d imagine that most of them coughed up a few extra denarii for a useful back-up weapon, rather than trust to fate with only a crappy spear.
But that’s just my speculation, I don’t recall ever reading an official reason.
This is the first I’ve ever heard about Triarii using spears. I wonder where Wikipedia got that? It does mention how the Triarii were the last ditch guys, so the spear may have been used in a shield-wall tactic to hold back attackers until the princeps and hastati were abel to rally.
Greater organic flexibility within the legion. A spear corps, as above, gave the legion a longer-range close combat force with more shock capability ( and incidentally better anti-calvary ability ).
Less difficult. It’s relatively much easier to learn how to properly grip and wield a spear and a long spear obviously gives you a longer range weapon with which to poke your enemy. A line of armored spearsmen, as they are back ten feet or more from the plane of contact, are going to do a lot more damage than the line of swordsman they are hitting.
That’s why Roman legions tended to buckle on first contact with Greek phalanxes - the phalanx was a better pure offensive unit. Unfortunately for the Gereeks it was also far less flexible and if they didn’t break the Roman lines quickly, the Romans would eventually begin to infiltrate the regid Greek lines and break them up.
What the triarii would do was charge at a moment of decision, to either shatter the enemy completely or shock them back long enough for the hastati and principes to reform and reenter the fray. They wouldn’t stay engaged long, unless it was a last ditch stand or they were filling gaps and poking the enemy from behind or between the lines of hastati and principes.
I imagine they might on a very small front. But lack of number probably precluded them from shielding the whole of a legion. More likely if the legion was pinned such that it had to face a directly calvary charge, they might more often have acted as backing, possibly with spear points extended through forward lines of hastati/principes.
The earliest Roman armies, say from 600 BC to 400 BC, were all spearmen like a classical Greek phalanx. They wore greek-style armor. This is when they were fighting Etruscans and Latins who were similarly equipped. When the Romans began to encounter more diverse enemies like the Gauls and Samnites, they changed their equipment to better meet the new threats.
In 390 BC a huge army of Gauls sacked the city of Rome. After this, the general Camillus reorganized the army to include a front row of swordsmen. They were armed with Greek-style swords.
Around 275 BC the army was again reorganized around lessons learned in the wars with the Samnites, who fought mainly with javelins, and the invasion of Phyrrus of Epirus who used the Macedonian pike phalanx along with elephants. The number of swordsmen was increased to include the first two ranks. This is the “pre-Marian” army you are talking about in your OP. This is the army that fought Hannibal. This is also the period when the spanish gladius sword was adopted and the pilum javelin was developed.
Around 105 BC Marius reorganized the army to make them all swordsmen. This was in response to the disasterous defeats suffered at the hands of the German tribes of the Cimbri and Teutones. This Marian army was the army that Julius Caesar had.
I don’t have cites for any of this except to say that I’ve been studying the Roman army for many years.
The Triarii were older men who were past their prime. I wouldn’t expect them to be successful at sword fighting if the hastati and principes had already failed at that task. If it “came down to the triarii”, the battle was pretty much lost. Their role was purely defensive, forming a tight phalanx to hold off the enemy while the rest of the army retreated and/or regrouped.
The thrusting spear is a simple and effective weapon, not requiring much training, especially if used in close formations. It’s an ideal weapon for ordinary citizen soldiers (i.e. greeks) or older guys who can’t effectively fight hand-to-hand anymore.