I tried a flat board as a stretcher and it just didn't look right - it was lacking. Yes, it is certainly a little artistic flair. The convex curve of the top of the shelf is mild enough that it can still be used as a flat shelf in most cases (just don't put a full glass of water on it).
The purpose of the stretcher was to add visual mass and another element of interest.
I painted it so that the grain of the wood took centre stage. Black makes the base seem heavier and more massive.