I think that Christians oftentimes added pagan elements to their art because the pagan symbols were sort of universal to non-Christian and to Christians alike because even the Christians were probably converted at some point. It was a way to grab pagans’ attention. In grabbing their attention, the pagans also needed to be able to understand what was going on in the images because that was the main way for them to learn. When missionaries today go to another country they don’t just babble in English and expect the people to understand them, they learn the language. Since many people who would have seen the images probably couldn’t read, it is kind of the same concept of communicating in a manner that they would understand.
Also, since the pagan elements were sort of universal, they could have been used to hide the true intent of the piece of art. At certain points in history Christians were hunted and oppressed so to keep their faith alive they had to hide and worship in any way they could think of. By adding pagan elements to their art they probably were able to conceal the true intent of the religious nature of the piece from official eyes.
One disadvantage of adding pagan elements would be inconsistency. If people expect Christianity to be a completely separate entity from other religions including paganism, then seeing pagan imagery could confuse people. So this uncertainty could arise straight from the blend of pagan imagery and Christian imagery. Although the Christians had good reasons for including the pagan imagery there is always a possibility of backlash and negative consequences.