Logical reasoning is very important when you are trying to ensure that the audience understands your message. If your message is not logical, people may consider you untrustworthy. Being logical can sometimes stall your opponent, as they try to unravel the bizarre sense that your argument makes.
Good Example:
Men are mortal.
Ben is a man
Therefore, Ben is a mortal.
Since Ben is a man, he is also a mortal.
Bad Example:
All men are mortals
Erin is a mortal
Therefore, Erin is a man.
Just because Erin is mortal, she does not have to be a man.