Our clients are reporting to us that during their visits to MIT a number of admissions committee members are "strongly encouraging" applicants to submit Sloan's Supplemental (Optional) essay. This is advice that should definitely be considered, but only if you really have something to say - not just a rehash of prior content or an ill-advised attempt at a goals statement. At a high-level, the supplemental essay is a test of your ability to deal with ambiguity. So consider this a de facto requirement, but (again) only if you have a different dimension of your character to bring to the table. If you can pass that sniff test - read on.