The assumption from MBTI is that our functions are balance and compliment each other, but not everyone is balanced. These are people who may excell in one area, at the expense of others. TiTe (TeTi) could be great philosphers, scientists or scholars. NiNe (NeNi) could be visionaries or shamans. SiSe (SeSi) could be great athletes or artists. FiFe (FeFi) could be great diplomats and mediators. On the other hand, not having a balancing function makes life more difficult so they also stand a good chance of being people with "great potential" that never seem to make anything of their life.
If your dominant and auxilary are both i or both e, than you may simply be very introverted or very extroverted, but otherwise balanced.
It could also be that people who are "unbalanced" in their two strongest are more pressured to develep their 3rd and 4th function, perhaps developing them at an early age out of necessity.
Moving to my cognative processes test as an example:
Going purely by the two highest I am NiNe. However, I scored "excellent use" in four functions: Ni, Ne, Te, Fi. So I am not relying just on N, I have a strong Te and Fi to back it up.
Unlike hard sciences, such a physics, in social sciences like psychology, an exception does not make the theory invalid. Take the theory of child development (stages, time tables for development, etc). Child geniuses violate the theory. So do children who don't develop normally due to mental retardation. But the theory is still true for most children, so it is still a valid theory.
So a handful of exceptions do not necessarily invalidate the theory, they just remind us that some of us may be "exceptional" (i.e. an exception) people. Now if 300 people posted to the thread saying they were also exceptions to the rule, that could be cause to question the theory.