Bret Devereaux does touch on an interesting concept though in his articles on this nitpick... That movie goers are so acclimated to seeing how "movie Roman" armies fight, that showing them a more historically accurate battle scene would be highly jarring to the point of being novelty.

I'm hoping the days where movies show the Romans forming testudo, moving up to fur clad enemies and then breaking up to do one on one gladiator style individual battles are gone.