Is the Short Sword and Shield Overrated?

[M. CVRIVS ALEXANDER]

So what sort of formation does it take for the Roman sword to be effective? A couple of sources such as Polybius have mentioned Roman soldiers actually being spaced 3 feet apart to either side so that they could swing their swords.

I believe he actually says that there is 3 feet of space allowed for each man, meaning they are reasonably close together with only small gaps between shields, but enough elbow room to fight effectively.

Actually, Polybius does say that the Romans were spaced 3 feet apart in all directions in addition to the 3 feet occupied by a soldier. Polybius is contrasting a looser Roman formation to that of the Macedonian phalanx, where indeed only three feet of space per a soldier was maintained:

18.29.1 That when the phalanx has its characteristic virtue and strength nothing can sustain its frontal attack or withstand the charge can easily be understood for many reasons. 2 For since, when it has closed up for action, each man, with his arms, occupies a space of three feet in breadth...
18.30.6 Now in the case of the Romans also each soldier with his arms occupies a space of three feet in breadth, 7 but as in their mode of fighting each man must move separately, as he has to cover his person with his long shield, turning to meet each expected blow, and as he uses his sword both for cutting and thrusting it is obvious that a looser order is required, 8 and each man must be at a distance of at least three feet from the man next him in the same rank and those in front of and behind him, if they are to be of proper use

So, no shield wall in the Roman army that Polybius has witnessed. References supporting the "shield wall" view would be greatly appreciated.