UrRong - the Rule of Threes is a great tool for creating an effective argument, but it is not the only option for illustrating a concept. Nikhil's point that two examples can be rhetorically sufficient still stands. Yes, three examples can provide a more fully-rounded argument, but sometimes brevity is more appropriate when discussing a concept in detail.
Take, for instance, Nikhil's original essay. In only two points, he made a clear and succinct argument about the effectiveness of two examples in comparison to three. The essay was brief, but rhetorical and powerful. It demonstrated that two examples can be sufficient to typify a concept without sacrificing quality or content.
In addition, Nikhil's conclusion was spot-on. Opting for brevity has got to be worth something in some cases. Not all concepts require a full three-point argument and sometimes two examples can effectively illustrate a concept without sacrificing any detail or nuance.
So, while the Rule of Threes can be used to create an effective argument or a funny joke, it is not always the best way to illustrate a concept. Furthermore, two examples can be rhetorically sufficient and brevity can be a valuable option in certain cases. In short: UrRong! The Rule of Threes should not be supplanted, but it should not be seen as the only option either.