JarlFrank
I like Thief THIS much
I'm sure this is correct with vikings and some light small shields but i have my doubts in case of Romans. Their ranks could get tight which at least in some circumstances would suggest using shield for more passive defence. OTOH roman line was supposed to be more flexible than others from the era and they fought in a less concentrated formations so it would fit. Possibly the way shield was used depended on circumstances?
Vikings also fought in shield walls a lot.
Using your shield actively is useful in formation, too. Romans especially used the shield in a very flexible manner, it needs to be mobile and easy to move around in order to form formations like the famous testudo.
You gotta be able to lift it over your head as soon as the centurio gives you the command.
Also, despite their larger size Roman shields are still light enough to be easily wielded with your hand, no need to strap it to your arm. The most common offensive use of the Roman shield was to hit the enemy shield in order to unbalance his defense, then follow it up with a sword stab.
The Roman way of fighting relied heavily on a core of flexible legionary units that could both operate in small squads and in larger shield wall formations. They managed to defeat Hellenistic phalanx armies with their more rigid formations that way: present a closed front but split off some lads to go on flanking maneuvers.