Town watch are pretty good for handling small bands of rebels. I don't expect them to kill, but I do expect them to hold a bunch of slaves in place while I flatten the area with arrows and cavalry. There's a tactic out there that calls for peasants as bait for missles or horses, but that's trying to hold back a tank with a water hose. Hastati are decent against barbs as I can build principes and triarii but for economic reasons(I'm Julii) It's not cost effective to recruit and ship to my front. I notice the AI in Custom doesn't sent hastatii against phalanxes either. It charges triarii, if it has them, instead.

I think one reason that the AI breaks it's units like that is a tendancy to use a unit like another unit if that other unit is absent. It doesn't seem programmed to do the best with what it has available. It's tactics may be based around a full well rounded army even when it has none.

Who knows it may be using tactics for one formation(in the formationsAI file) butsits short on a certain unit, so it's improvising.