Have to take issue with the 'experienced climber on a hard pitch wouldn't stop to instruct a beginner' comment from terrymorse. If it was an outright safety issue, they surely would. In fact, some years ago I was watching a guy lead a hard route with sparse protection, thinking he must be completely concentrated and unaware of anything but the task at hand when he suddenly called over to another party "Hey! There's a knot in your rope!". End THAT illusion. If I saw someone about to rappel off the end of their rope, I assure you I wouldn't hesitate to inform them of that fact, even if it meant 'blowing the onsight' of the climb I was on.
On the other hand...I fully agree that there is a right time and place for beginner level instruction, and it isn't mid-climb, nor mid-paceline.