I think what we should remember here is that supplemental feeding is intended to be supplemental. Deer will seek out what their bodies need, and will select for high protein forage if their energy requirements have been met or bolstered by supplemental feeding.
If someone is fine with and can afford to foot the bill for protein in their supplemental feeding rations and they are happy with how that has been working out for them, then there is nothing wrong with that.
Also keep in mind that just because a feedstuff is not classified as a protein feed by nutritionists, it doesn't mean that it cannot contain more than a moderate amount of protein.
For example: whole corn --> 10% Crude protein content (That is a pretty good amount of protein)
Now lets look at what is considered a protein food item...
Soybean meal --> About 54% crude protein (You can see why its classified as a protein item)
So if you are feeding Corn + a low % of soybean meal, then you would be offering a great deal of protein for supplemental feeding. (I would never recommend this in a feedlot / pen situation due to the risk of acidosis occuring)
The deer should be able to fulfill the rest of its protein requirement from the land, which they are forced to do at a level of approximately 6 to 8% anyways so that they don't die as a result of their microbes dying. Most of the time they should be able to find between 10 to 13% of their protein requirements.
Here is the resource that I find myself referring to from time to time. It is a little long, but I wouldn't recommend it if I hadn't read it myself and found it useful.
If antler growth is important, then page 93 may be useful.
http://texnat.tamu.edu/files/2010/09/supplemental_feeding_deer.pdf
