Wasn't Carthage's infantry inferior to Rome's to begin with? I recall from a book regarding Cannae... Hannibal was sure enough that his center, composed of infantry, could not hold up to the immense push from the Legionnaires. He counted and planned for his center to bend, but not break, so his presence was required there to prevent a disaster. His cavalry on the other hand, which were on the flanks were quite excellent and cut apart their Roman equivalents and "sealed the deal" by attacking the rear of the Romans.

I haven't tried playing as Carthage, but from the occasions that I've faced them with Julii... their infantry can be easily managed by Roman infantry, but their Cavalry is quite deadly against Rome.

A quick, simple use for Carthagenian infantry is just meatshields for the Legionnaires, while the true punch of the Carthagenian Army is the Cavalry and a few Elephants.