I'm not an expert by any means, but I did ask one a while back about this. Here's what he told me, at the risk of peeing off my main sausage stuff supplier (names have been omitted to protect the innocent - me!).
What he said was, adding powdered milk solids to add flavor or 'tang' is a scam designed to market surplus whey/buttermilk solids, that they don't actually add much of anything. I've used Fermento, principly whey protein and dried milk solids, and, while it didn't hurt anything, I didn't particularly notice that trademark tang either.
If you're not going to go 'long form' and ferment/air dry sausage - using Cure #2, fermenting cultures, etc, the only way to develop that extra zip is to use encapsulated citric acid. Its encapsulated so that the citric acid isn't released until the sausage reaches a certain temp. Apparently, if its released too early, or interacts with the meat too long before reaching the finish, it can drop the pH of your meat, causing high water loss and dry crumbly sausage.
Like I said, this isn't me talking. Hopefully other members will chime in here with their experiences using powdered milk or the ECA. It could be the powdered milk is also useful for retaining a certain amount of moisture, or for binding the meat.