The purpose of the decree was twofold: ease of bureaucracy and to help fill Rome's near empty coffers.
What Caracalla failed to understand, and to the detriment of an already declining society was that the people he made citizens; Moesia, Dacia, Pannonia, supplied an endless resource of able bodies who have had at the time, no dealings with Rome itself. Not only were they unfamiliar with Roman custom's, insofar as everyday Roman life and the worship of Roman gods, they had little knowledge of the universal language of the empire: Latin.
This lack of attachment to Rome, and the relevant non-allegiance to a city hundreds of miles away allowed the infiltration of foreign ideals and norms into an already established Roman culture, which had been in existence since the early 700s BCE. The "1000 Year Empire" as it styled itself to become was already in it's final death throes before universal citizenship.
In 21st century America, with the ongoing debate of illegal immigrant status and universal amnesty, a parallel is being drawn. There are the many who seek advancement in America through legal processes, and others who believe they have a right to be here, legal or not. Immigrants are the backbone of the United States, and keep the blood of the county refreshed. New ideas and innovations came with immigrants of the 20th century, and they assimilated into an already established culture.
The early days of immigration to the United States saw the influx of people from various nations in Western Europe: England, France, Italy, Germany, Holland, Spain and Ireland. During the Slavic upheavals of the late 19th/early 20th centuries due to war, famine and disease, the face of the immigrant changed to that of the Pole, Croat, Lithuanian and Russian, in addition to many other cultures across Eastern Europe. The people who settled in the large cities of America did so in their own small communities, keeping their heritage and culture alive. After a generation though, they began to move out of the cities and into suburban areas. Though some retained their languages, all became Americans. They pledged their allegiance to their newly adopted country.
From the mid 20th century until the present, the largest immigrant population has come from Central and South America, with Mexico leading the pack. Values changed, and indoctrination has taught the newly arrived immigrant from South of the Rio Grande that they no longer need to "lose" their culture and become Americans. They have their own culture, and should be proud of it. It is not they that need to conform to America, it is the Americans that need to conform to them.
In retrospect, mass citizenship to people who have little to no allegiance to the nation/empire granting the citizenship is like the Trojan Horse of Homer's day. The nation will tear itself apart from within.