I just pulled out my monitor and it is interesting to note that the MM degree requires officers who are not of those and those alone that are required, particularly the SD. Not to mention, I think you'd have one hell of a hard time carrying out the third section with only the number required.
I think what it really boils down to is that over time, the order went from a secret society who only were accountable to the old charges to one with bylaws and far more rules but who didn't want to give up the old ways. I'm sure at one time it was basically "keep Fred on the lookout. Oh, it's only us? Okay, well then keep your voices low". At some point in time, the obligation changed to include the part about being beholden to the Grand Lodge. From that time forward, things got complicated. No longer could the rules simply be what was in the obligation, they then extended to whatever the GL said. It puts us all in a weird position of swearing to an open ended set of rules.