[WingedDog]

They could buy you some time. For example: the large army of your opponent is about to take a province of importance. Your main force wouldn't make it in time to protect it, but you have send there a small army of glaiators from your castle. Your opponent has a tough choice: either to fight gladiators combined with PD and take heavy losses or even to be defeated, or send there a scout to kill all your gladiators, but lose a turn, and next turn your main force will be in the province and would give him even harder fight. You manage to keep the province each way.

Summary: gladiators are strictly situational units that could find ther niche in the game rather for the defence then for offence.