There isn't one single book that exists, which states we can't use all 8 functions. But there are plenty of books that do exist, which tell you we CAN use all 8 functions. Frankly, what is telling to me is not what someone claims as an MBTI type, but rather how they choose to utilize the theory itself. With some, I've never seen such an example of destructive use. Telling people their functions "suck" or they "use demonic functions" or "the world would be a better place without X," well, what they are really doing is subconsciously choosing to disown a part of themselves they simply do not like. I don't understand why all the hate. People can't hate a function, when that function is a mental process within everyone, including themselves.