Audio sync for Avid


My question is directed to Larry C. as i know you were a former editor.
We are routinely asked to create Avid media with the avid setup in a film project 4 perf so that the assistant can perf slip the sound.
My question is this if we are properly putting the sound in sync on the cortex which to the best of my knowledge allows sub-frame slipping why would the assistant editor ever need to perf slip?

Making sure @larry sees this.