I believe that a source of protein being present isn't intended to rebuild torn fibers, but to keep your body from metabolizing healthy muscle tissue after a certain number of hours of activity
Whew, after some of the other posts, I wondered whether anyone had an accurate understanding of why dietary protein is a essential sooner or later. Dietary protein, if it is a "complete" protein will contain all the building blocks necessary for the body to assimilate and transport into fatty-acid proteins which eventually repair and maintain organs and tissues. (In other words, your body.)
Your body doesn't really "speed up" metabolizing muscle or organ tissue for energy, it just fails to "keep up" with repairing and replacing cells during heavy exercise.
Anyone interested in this sort of thing should understand that just because "dietary protein" is good for you, that it somehow helps exercise performance as well.
Whether or not you benefit from adding dietary protein to your exercise diet is not predictable across a range of exercise-time-frames and differing exercise intensities.
I suggest the protein containing exericise-diet drinks are a good training aid, but do not increase exercise performance in events lasting less than 8 hours. However, biased studies state otherwise.