I disagree with the author. Bringing up "what about x" has many valid uses. For example:

"Look at my perpetual motion machine!" "What about the cable plugged into the outlet?" "Don't change the subject!"

"You are a murderer because you killed Mr. Jones." "What about the fact that Mr. Jones broke into my house and was trying to rape my wife?" "Two wrongs don't make a right!"

Certainly it can be abused but very often the "what about x" argument can be a valid way of introducing new and relevant information.