The +1 EV matched (more or less) the look of OOC JPEG with high-dynamic compression enabled (on Nikon, it’s called Active D-lighting, or Auto Lighting Optimizer on Canon). This is suitable for landscapes and night scenes.
But then, studio shots (under controlled lighting) were over-brightened compared to the camera base curve (OOC or darktable’s). So, after discussions, we chose to tweak the defaults to match the most basic case (hence +0.5 EV), assuming the people doing HDR-scenes will most likely be technical enough to fix the brightness themselves, while the people shooting with basic settings are the ones who most likely need their hand held.