It is vital that we remember that the Roman Empire had grown substantially by conquering many different cultures, races, religions and languages different than their own. This situation created an extremely diverse population that the government had to teach and make these other cultures somehow fit within their control. The changing society challenged the multiple gods theocracy in favor of a singular god belief system.
It seems to me that there are far more reasons and advantages for the government to blend Pagan and Christian art elements. Using some familiar Pagan elements or art and even doctrine can help engage those who previously believed in Pagan theology. The combining of these earlier elements will make the story-telling and teaching easier since we'd expect people to have more interest in something they already understand or have been taught. People may start out challenging the new theology, but once they have been "re-educated" they are more likely to accept newer explanations and beliefs based on more current knowledge and those influencing their thought processes.
People are more likely to change slowly rather than change drastically. So it would be wise for the facilitators of change to pick and choose the elements and even doctrine that they could influence the quickest. Then leaders could revisit or challenge other aspects of the previous beliefs once followers had joined the wave of change and new ideas. In addition, utilizing the best architectural art elements of earlier civilizations is always wise in building a structure to last. Take ideas of the past and improve upon those elements to reach progressive building concepts.
A disadvantages of blending the two theologies is that some more contemporary thinkers may choose to dismiss Christianity because they have already decided to dismiss the earlier Pagan theology. A couple additional incorporated elements are the basilica, sarcophagi. The Basilica became the Christian model for churches- the advantage is that the public would fill more comfortable in a space and environment that they themselves had already experienced. Thus, they would less distracted by the newer elements. Cremation had originally been Pagan but was rejected by Christians. Thus, the wealthiest people, both Pagan and Christian chose to use elaborate sarcophagi instead. However, universally converted from Pagan and Judaism all strived for salvation and a heavenly afterlife via the sarcophigi. Using the Late Empire art element of more motion and activity in art would help involve and/or intrigue citizens in an almost entertainment method. Also using Late Empire the technique of less perfection in detail and anatomy might actually serve to make normal, average people identify as more similar to the subject matter.
Ultimately the contempory Roman governement was wise to try to include and blend some Paganistic elements with newer Christian art and theology because it help convert and re-educate much of the massive, diverse and growing society.