It's so their tools are easier to clean, and also so they can get more work out of the deal in a few months, when the plaster starts falling off of the wall.

 

It makes the joint compound smoother, if you don’t add too much.

 

That makes sense, of course. But does any potential benefit outweigh potential problems?

My "default thinking" on everything like this, is to ask what the professionals do and don't do. That carries a lot more weight than a how-to video.

 

IMO if it had long reaching good effects, the manufacturer would either include it in the mix or leave instructions allowing it to be used and for what purpose. Snake oil.

 

the manufacturer would either include it in the mix

Not necessarily. They make the basic product, which works well. Adding anything will increase the cost. Some finishers have found a little soap gives a better finish. Some even add plaster of paris to the mix, which gives a harder finish.

 

I would not add anything to it that the manufacturer did not recommend or approve, the only exception being water depending on what you're using the compound for, since the compound already has water in it, and it can evaporate of course, so the amount of water in the compound is technically not set in stone.

 

That's my gut feeling.

If the manufacturer knew that it was advantageous to shoot some Palmolive in there, why wouldn't they do it themselves? 2 cents' worth in each 4.5 gallon bucket isn't going to add much to their cost.