This is actually modeled pretty well in Mythras. Javelins have the unique property of being able to Pin Weapon (Shield) - effectively rendering it useless. If one were to be in a shield wall, you would be able to Passively Block one more location (5, presumably arms, torso, abdomen, and head), making you fairly hard to hit with arrows. A javelin, however, could rip apart that shield wall by putting in gaps in the wall.One of the things the guys over at Regia Anglorum found was that an arrow would not penetrate standard Saxon shield but a javelin would go right through the shield with plenty more energy to kill. How ever you could easily dodge javelins but not arrows
This is what makes what sort of infantry formation you were in important. If you were in closed formation with many friends around you it was possible to hide behind the overlap of their shields but if you were the foe threw javelins you had it. If you were in open order you avoided the javelins until an archer brought your unprotected arse down.
When in a dispersed formation, you won't be using passive blocking as much in all likelihood, and the shield will be more actively used. This, in turn, makes it less likely for the pinning of it to happen.