The diversity of personality theories allows evolution in the thinking about personality to occur. If all psychologists thought of personality in the same way, the ideas behind what forms personality would never change. The change in thought over time allows psychologists to change and adapt their methods of counseling around a patient's needs rather than making assumptions based on information that later proves false.
A psychologist (or any professional working with individuals) who ends up looking at personality typing theories or turning to personality as a factor in behaviors has a wide range of knowledge and ideas from which to draw. This allows the psychologist to consider several potential theories and determine the best fit for the situation rather than following a single guideline that is not necessarily appropriate for every individual.
Though theorists approach their theories from different angles, they may all experience similar pitfalls. For example, the theorist who grew up in an area with minimal diversity would form thoughts and opinions based in a lack of diversity. This point of view might not be applicable to anyone from a different socio-economic status or even someone with a different cultural and ethnic background. The theorist also makes assumptions based on his experiences, which are not always applicable to someone who has not experienced the same situations.
With a diversity of theories comes a diversity of languages, nuances, slang and phrases. The differences in language can lead to misunderstandings or mistranslation, which confuses the theory or changes the application of the theory. For example, Freud's original works were in German and were metaphorical, but translations into English make him sound scientific and literal instead. Even in the same language, there are potential misunderstandings due to slang or differences in how a word or phrase translates to the mind. For example, the phrase "his bark is worse than his bite" might apply to a person who seems threatening or intimidating by his speech but is unlikely to take any action or to a dog who has a deep or scary-sounding bark but is actually a small dog.