I've noticed that when i think of Romans, I think of tall shields and short swords. It seems that a phalanx with spear-like weaponry would have been more efficient. If the Romans did abandon this form of combat, I'm curious why and how it transformed from spear-like weaponry to using short swords.
From the time that the Roman army became more than just a rabble of soldiers, they in fact used the typical Mediterranean phalanx system. This changed during the Samnite wars (there were three of them, between 343-290 B.C.) in no small part due to the simple fact of terrain. The Samnites were an independent confederation/people group who lived in the Appennine mountains in south-central Italy. As Rome expanded, friction between Rome and Samnium eventually led to war, but Samnium is a rugged, mountainous area and the phalanx is only effective on a flat, featureless plain.
Think about how a phalanx works- you're shoulder to shoulder with your comrades, marching in lock-step with the men on both sides of you. Even a tree or particularly tough bush could break up the formation if a couple men have to split to go around it, and because of how tightly packed the men in a phalanx had to be, a broken formation is a deeply vulnerable one. Rome's phalanxes in Samnium were defeated time and time again by the light troops of the Samnites because they simply couldn't handle the rough terrain- they stumbled over rocks and steep slopes and became disordered; easy prey for skirmishers and light infantry.
The maniple system, or a "phalanx with joints" was a solution to this problem and in fact, the flexibility of this formation served the Romans very well against the phalanx-based armies of the other Mediterranean powers for the next few hundred years until being supplanted by the reforms of Gaius Marius, in reaction to the Gallic armies Rome faced in his time.
As far as the gladius, the quintessential Roman short stabbing sword, the first iteration of this weapon (the gladius hispaniensis) became common in the legions around 200 B.C. Unfortunately, I am about 20 miles away from my book collection on the subject, but around the time of the Samnite wars I believe the Romans would have still used swords of Greek or Etruscan design such as the Xiphos, which would have been primarily short thrusting blades not terrifically unlike the later gladius.