I have played with Arcani and Gladiators purely as the initial assault troops when taking the battlements of stone walls. It's not so much their combat capability that I want but their small unit size; this makes them far more agile when manoeuvering since they suffer less from the delays caused by trying to cram an entire unit up a siege tower or through a doorway in or out of a tower.

By preference I use purposefully understrength cohorts, even to the extent of allowing a spare unit to be chewed up a bit in battle. As long as you keep them on the walls if there are any cavalry around, they rarely get killed off and they tend to rack up lots of kills when they do fight, standing them in good stead when I finally retrain the unit up to full strength.