When are personality tests useful?

I met a few training professionals who use DISC, a personality assessment to analyze people. They observe the person for sometime and then come to a conclusion about which personality traits (combination of D, I, S, and/or C) they portray. I asked one of them how this information helps them. He explained that knowing a personality helps me deal with the person at work. As a trainer, I answer a query/response based on the personality that my learner depicts. I also know that I need to have a high 'I' for my audience to 'like' me. They have to like me to want to listen to me. Makes perfect sense, doesn't it.

How are personality tests useful in learning? When can they be used? Here are my thoughts:

• Personality tests are extremely useful when you want the learner to reflect on his/her own personality.
  
• These tests are useful when you need to bring about a behavioral change. For example: For a salesperson, there are a list of attributes that he must have and others that are undesirable. We can use personality tests to check where the learner is, deliver learning, and check where the learner to identify if a behavioral change has taken place.
• These tests will definitely help us understand the learner profile better. We can design courses keeping the dominant characteristics in mind.
  

Some thoughts that follow these points:

• Does a behavioral change mean a personality change? Do we understand the deficiencies in our personality and consciously work on them? I guess, personalities are also outcomes of a person's socio-historic context (the environment). This would mean that our personalities are constantly changing based on our experiences.
  
• Will the audience show a specific personality pattern? This could happen. If we take the salesperson's example again. When a company hires their sales executives, they look specific characteristics features. How confident is this person? Will he/she be able to hold my attention for long? Will he/she be able to convince/persuade me? So, they may show a specific pattern. (If anyone knows for sure, please share.)
  
• Would it be more useful in classroom training? You interact with the learner directly. Based on how they behave, you quickly categorize them and respond accordingly. It sounds like a tough job.