Back in the 80's, my mother took my sister to Acapulco without a letter. They made it to Dallas before they were stopped and asked for the letter. We had been to my relatives in Canada many times without a letter prior to this, so it was something new to my family.
The airline finally relented and called my Dad. He described my sister, the clothes that she was wearing and the expression on her face. They finally let her through.
Now, for my nieces and nephew, we always have one. Most of the time, my sister has not been asked for the letter, but her kids are now older (1 tween, 2 teenagers). She figures that it is better to be safe than sorry.