I rented a car 3x in Argentina back in Nov/Dec. last year. Don't recall the IDP was required by the rental agencies (National and Avis) but it substituted for a passport. Also went through at least 2 roadside checks (entering Jujuy from Salta on a side road, and near Salta city coming down from the Chilean border) which were more to make sure one had insurance and were not illegals. Might have needed the IDP if stopped for a traffic violation (but it'd seem that would be rare in Argentina) or maybe an accident.