I was down with this theory for I long time before I saw this thread. Maybe as an extension to your theory, cutie marks not only determine what your occupation is, but work on a higher level by making you fill part of a complex social structure with little room for free choice. It makes sense, considering that it seems like all ponies have their place in society, and no-one is without a job. If you believe this to be true (and bearing in mind that it is only speculation), it reverses the whole idea of Equestrians' being different and special. Conformism at its finest.
I like being controversial. Good day!
Also... I have an eye, and the corner of my screen looks like a triangle... this must be the work of the ILLUMINATI