People who are foreign residents of Argentina (and other Mercosur countries) are not citizens, and therefore are not member citizens of the Mercosur (who are legally allowed to pass on their national identification card, without passports), and therefore are subject to the same requirements when entering a country where they are non-resident as if they had no residency; i.e. being a resident in Argentina does not make one a resident in all Mercosur countries, with attendant privileges.
Argentine foreign residents who have done so have been lucky in that the immigration officials at whichever border crossing either allow it out of lax border controls or ignorance of the difference between a regular DNI and a resident's DNI, or simply don't look to see the difference, or a combination of the above.
Much like perma-tourists who have no residency status here in Argentina, at some point someone attempting this would be denied entry on a DNI alone and personally, I would sure take my passport with me to ensure that I was not turned away at the border with the only possibility being a trip back to where I live in Argentina. Or paying a bribe to get across, with all its attendant risks.
As far as re-entering Argentina with just your passport - your passport is linked to your residency/DNI and they will know you are a resident when you enter with just your passport. Whether they ask you for your DNI is another thing, which I don't know, but they know you are resident here.