If the Dough Doctor says that anything under 5% dry milk addition will not make any difference in the dough why do I see small amounts being used on many forum recipes?
That is a very good question.
According to what Tom has said, there is some dough strengthening effect even when using below 5% dry milk. That might help if the dough is to be run through a sheeter or roller of some sort but I tend not to think that that is why some pizza operators use dry milk. I think that a lot of pizza operators just decided to add some milk to their dough, simply because they had it or maybe to satisfy their curiosity. They perhaps liked the results, or perceived such, and just decided to continue to use it and the recipe eventually became a family or legendary recipe to be handed down from generation to generation and guarded like it was Fort Knox. It might have also been used as a differentiating factor. For example, for years, Donatos boasted about the health effects of the milk in their dough (and eggs as well). They no longer do that. At Vito & Nick's in the Chicago area, milk (fresh milk) is a hallmark ingredient for their dough for their famous Chicago thin-crust pizza. I don't think they would ever dare to leave the milk out of their dough. Their customers would be picketing the joint. I might add that V&N uses a roller for their skins so the milk (at around 12% of the flour weight by my calculation) may have a beneficial effect for that purpose.
In Pizza Hut's case, I do not believe that they ever used dry milk alone, or at least I could never find any evidence of it. As I noted at Reply 1 at http://www.pizzamaking.com/forum/index.php/topic,8791.msg76201/topicseen.html#msg76201,
I believe that PH was using a dairy blend. The pdf link in Reply 1 is no longer active but you can see a typical PH dough formulation using the dairy blend for its pan dough at page 4 of the pdf document at http://www.espanol.pizzahut.com/menu/nutritioninfo/documents/ph_ingredients.pdf.
That is an old document but represented what PH was doing before it went to frozen dough for most of its pizzas (in the U.S.). Later, it appears that they abandoned the dairy blend but continued to use whey, as can be seen in this 2008 pdf document: http://www.pizzahut.com/files/pdf/pizza%20hut%20ingredient%20statements%20september%202008.pdf.
That document is after PH went to frozen dough. Pizza Hut has stopped publishing pdf documents for its ingredients so it is hard to say exactly what they are now using in their doughs. Since their current doughs are loaded with chemicals, that is perhaps no great loss.
If Steve (slybarman) would like to replicate the "old" and, arguably, "better" PH pan dough, without all the chemicals. he might consider using a dairy blend. Dutch Valley uses to sell it but I could not find it among the products at its website this morning. But I found another source: http://www.roundeyesupply.com/Land-O-Lakes-Superheat-All-Dairy-Blend-p/de127782.htm.
Or Steve can make his own dairy blend using the same ingredients. His kids will thank him for the added nutrition.
EDIT (4/20/13): For the Wayback Machine link to the above Pizza Hut pdf document, see http://web.archive.org/web/20100602083641/http://www.pizzahut.com/Files/PDF/PIZZA%20HUT%20INGREDIENT%20STATEMENTS%20September%202008.pdf
EDIT (12/30/2015): For an alternative link to a dairy blend, see http://www.roundeyesupply.com/Land-O-Lakes-Superheat-All-Dairy-Blend-p/de127782.htm.