With the SPQR mod there are 63 additional cities on the campaign map, and it's almost impossible to have a turn without any sieges in progress. The AI maintains lots of large armies, and they may get put to more use with no walls. I've started a campaign with the SPQR mod and Pode's no walls idea, but I left the stone walls on the 6 cites that have them to start: Rome, Carthage, Syracuse, Corinth, Athens and Memphis. No further wall building will occur. Carthage is a powerhouse in SPQR mod, and already captured Messana from Scipii in 268 BC.