Hi Larry!
Wives can be tough sometimes.....mine made me give up smoking for a year!! The food was ALWAYS late and mostly unedible. This was many years ago and she has fina;lly come around to enjoying the food's I cook....funny what a few winning ribbons wil do!
Anyway, the reason it does not dry out is because pork butt is about 25-30% fat. Hence the huge amount of meat loss when cooking. Now, also keep in mind, after it is "officially" done, I choke the fire out so it mimics a warming oven.
You are very corrrect in that doing this with poultry would dry it out. But that is a very lean meat.
You mentioned beef......for long cooks like this, you need to use a fatty cut of beef. Brisket is very familiar but also chuck roasts(the fattier cuts) work great using this method.
The leaner cuts of beef...steaks, rounds, etc.....would aslo dry out using this method...much too lean to survive long periods of warming time.
I would encourage you to try this! You MUST use a large one, 8-10 pounds, for this method to be effective.
General Rule of BBQing..........fatty cuts of meat should be slow cooked, lean cuts of meat should be quickly cooked.
Hope this is helpful!