I second the point from Algorithman : I totaly agree. I find C++ is really powerfull for production-grade and optimized code, and UE4 API relieves from some C++ hasles. But R&D on prototyping complex logic may generate the BP spaghetti effect and it very time consuming in C++.
More over, you may do some prototyping outside of UE4, in some standalone console or WPF application in C#, because of its productivity-gain, or when you need an context-free testbed. Then, when come the time to go from R&D to UE prototype, it would be cool to test the UE4 integration of a component with minimal conversion, before investing time in BP or C++ translation.
I personally have a few components, engine-agnostic, that I used in various environment (standard Winform, or Neoaxis Engine for example), in C#, which I will have to recode in BP or C++ just to integrate at prototype stage, to test the usefull-ness (procedural algorithms, some specific math, etc…).
For this reason, I thank you, Algorithman, for your klawr initiative !