This is exactly the SA way at present... they do have enough power tight forwards for almost a full change at 40-50 minutes. I can't work out if you think this is good or bad for fatigue? A lack of full match fitness should the need arise to play a full game (e.g. early replacement due to injury), or low fatigue due to reduced game time.
There is certainly a risk having only 1 back reserve... they have a scrum half on the bench this weekend, Faf can also play 10, Willemse can move around and also cover 10... there is enough flexibility when you also have the option of keeping it in the forwards, who will most likely be dominating for the full 80 minutes.