Even for three (full) days in NYC, 3 round trips and/or 6 boardings is not unlikely if you're a pretty active traveler. When we go, we're often all over the place - I think we've probably never "left value on the table" with unlimited ride Metrocards.

An added benefit is that if it's easier (because you're tired, it's raining, or you just want to save time with a short hop) to hop the subway for a short trip, you don't have any worries about waiting a full fare to just ride one or two stops.

At this point, I just automatically go for the unlimited if I'm in the city for three days or more.

(I should note I'm also a pretty dedicated public transit user, so this may not apply if you're inclined to go the taxi/Uber/Lyft route easily.)