Nearly all of the protein degradation happens at malting, specifcally the germination step. When brewers talk about 'protein' rests, they seem to be talking directly about proteins. In all actuallity, it should be called a 'Nitrogen' rest. Yes, protein is the number one supplier of N2. However, most people fall to realize that starch goes to dextrins and Maltose anywhere from 14:1 - 10:1 ratio which is a MASSIVE degridation. When it comes to protein, we need to talk about solubilized N2.
Scientists have done tons of these types of experiemnts, basically using distilled water and mashing at 45C for 2hours. Their control is the same mash but using an protein inhibitor to stop all enzymatic activity. The difference is the soluble N2 in the malt itself. When you compare the malt N2 with the active sample you'll find a 1:1 - 1:1.6 ration of soluble protein. This is all dependent on HOW the malting was carried out much more so than the variety of malt used.
If we talk about proteinacious matter, we would see a 1:1 - 1:0.6 change. As you can see, this is VERY limited.
Going even futher, only about 1/3rd to 1/5 of this availible soluble N2 actually solubilzes and goes into solution of the final beer. This is taking into account hot and cold breaks.
Now we get to the sexy bit. Eventhough there is a very small amount of the total N2 that goes into solution, it is VERY important to the final beer. The hard thing is that each style is different. A style high in IBUs will have a ton of polyphenols that will form complexes with proteins and precipitate out. A beer that is fermented very high, around 158F, that doesn go through a rest at a lower temp will have a much more flabby pallate and mouthfeel than one that was.
The major players?? The medium size proteinacious fragments that one gets from rests anywhere from 35C - 60C...these are the players.
So what should you conclude?? For beers that are very straightforward, US/ UK ales and such, dont really need a protein rest b/c the malt character and mouthfeel arent that important for the overall character of style. On the other hand, very simple beers like single malt lagers, very much need it to get all the character possible out of the malt.
If you dont, will you get a bad beer?? Of course not. On the other hand, whats 20 mins or so.
One more note. No one doubts the effects of melanoidins for the flavor, aroma, color etc for beers. Most rely on the specialty malts to give this effect. Melanoidins are basically an amino acid combined with a sugar. When you look back and see the limited change of soluble N2 has and think about what effect can it possibly have...this is where it comes into play. Melanoidin fodder. That little change, when combined with heat and sugars, can and does have a dramtic effect to your finished beer. Both technically but most importantly, hedonically.