Well, the Antesigniani are described as being a more mobile type of infantry than the Legionary line troops (the description says they wear the bronze quirasses to avoid the greate rencumberance of the long Lorica hamata, remember). I actually modded mine to use the semi_fast skeletons to represent this. Anyway, given that by the description they're apparently supposed to operate away from the security of the main battleline giving them the option to go into shieldwall might actually make sense, as an emergency cavalry-counter or similar that is easy enough to train the soldiers in.