The benefit of multiple days is indeed that adding another day is not even close to the same cost as day one or two. So if you can squeeze in half a day, especially at a park with morning EMH and near your resort (if all the stars align!), then definitely worth considering. But also know that you can wait to make that decision until near the end of that trip when you will have a better feel for whether there are still things you really want to do (or do again).
On the flip side, if we know we're in that position of needing to leave by noon or early afternoon, we tend to defer any major resort pool time or Downtown Disney exploring to that last morning and just hit the parks pretty hard on the other days. Nice to have a few hours just to chill out by the pool at a time when we otherwise feel like we need to be in a park to be getting our money's worth. Other options might be water park, mini-golf, bowling at Splitsville, renting a boat on Bay Lake/Seven Seas Lagoon, or exploring resorts -- i.e., activities that don't have to take a long time but you're not prioritizing during the main part of your trip. My wife is actually looking at letting our kids take an archery lesson at Ft. Wilderness this summer, which is something I didn't even know existed. In short, plenty of cool things to do -- some "free," some not -- outside of the parks.
