Black and white thinking is a barrier to effective communication skills. Too often we discover how we’ve limited ourselves and flip into the opposite view. The “but” hunt you hear among motivational and communication professionals is an example of 
good observations gone bad by over-application. (“How big is your ‘but'” and “‘but’ negates everything that went before it and should be replaced with ‘and'”.)
The result often is unclear and convoluted speech where people overlook legitimate concerns because they don’t want to have a big ‘but’ and where people awkwardly use other words to say what ‘but’ says perfectly well.
If I say,
- I’d love to come, but I already made plans
…the use of the word ‘but’ does NOT negate the fact that I’d love to come, and is a much clearer way to say no than “I’d love to come and I made other plans”.
The point of this post is not so much about how you need to stop the ‘but hunt’ as it is about how effective communication skill involves taking what you learn and asking if it helps you communicate what you really mean. If it doesn’t, tell the professional who gave you the rule, “Thanks, but no thanks” and opt for clarity.