Do you have an example of such a movie in mind?

It's meant as "othering"- "we" are the side of honor, organization, loyalty, hierarchy, cleanliness, and art (things that our modern society values), "they" are the side of vulgarity, barbarism, cruelty, ignorance, and filth (things our society does not value).

It also helps that most movies have the Romans speaking with a British accent- a lot of the "we are like Rome" propaganda reached full steam in the 19th c as Britain expanded its empire. Romans were bringers of light, knowledge, art, etc, and the world went to hell without them-- so too the British Empire is bringing you technology, government, and society, and you just didn't know your world was terrible before.

For examples of this kind of "redefining" history, see http://catdir.loc.gov/catdir/samples/cam032/99087438.pdf , and also note the corresponding fashion for classical monuments during the period of greatest expansion.

Also, on a related note, the association with the British and Rome vs the barbarians really reached a new level in the early 20th c- previously, it was more like Rome gave a spark to the inherent glory of the native British people, after 1900 (when the German government started using Teutonic propaganda to trumpet the "master race" (Wagner)), it was us against the barbarians again. Except that these barbarians were Germans, just like in the third century. Apologies for being overly parenthetical, it's early.