I don't understand perfectly a difference between the plain form and the casual form in Korean that you talked about. I think the plain form is like this; "안녕하십니까." Contrary the casual form may be like "안녕."

If my guess is right with your thinking, I would tell you the questions: first, when a person usually or only uses the plain form than the other form, he/she seems blunt and masculine. That may be true. But in current times, we've gotten the equality for gender so that many Koreans don't see the form is 'masculine', and it is a mind you have to own always. Second, I never know about '어/아' form. Third, the plain form is used less than the casual form at talking. It also equals with your guess. Fourth, it's a fact as well. Last, the question may have no meaning because ways to hide our emotions, feelings, thinking are very plenty.