Weighing the effort against a short trip.
Even for a short trip, if your destination requires or commonly expects an IDP for foreign drivers, the length of your stay doesn't change that requirement. A rental counter or checkpoint interaction can happen just as easily on day one of a three-day trip as on day ten of a longer one.
Given how simple the process is, most travelers renting a car abroad find it easier to just have the document on hand regardless of trip length.