That isn't a 24" rear wheel in this photo, it is a 26in. It is looks smaller than the front because the front is a 650b. I did run a 24in in the back with a front 26in a few years back (see www.pinkbike.com/photo/8612193). I don't follow trends which is why I have been running wide rims and an inverted fork for years. I used to run a rear 24in because it allowed me to run a larger tire on a stronger wheel at a lower weight. It gave me more traction but kept the wheel diameter lower than the equivalent 26 version and thus made the bike accelerate better and made manualling easier. Contrary to popular belief running a smaller rear wheel isn't a disadvantage seeing as you don't hit bumps in the same way as the front.
I now run bigger wheels because I live in a area that requires more pedaling and that 24x3.0 rear wheel does increase drag.

 Why the 24" rear wheel? I thought this trend died long ago...