Gladiators are supposed to be a hard hitting unit with loads of stamina. But their stamina seems to run out really quick....I assume it's that hardiness bug or whatever that makes hardy units get knackered even quicker than normal.

They should be useful for, as said, wall defence (because they hit like freight trains until uber post-Marius units and don't get knackered, so can keep on fighting effectively), and for fast reactionary infantry on a battlefield. They can run anywhere you want them to and still be able to lay the hurt down. On a siege attack, they can be good breach stormers, hard hitters who can get in there, really tear the place up and last long enough for others to arrive.

They have fairly limited uses, but gladiators aren't utterly worthless. Well, I suppose the Velites ones are stinky....

I vote for incendiary pigs.