I think the idea of being able to develop both kinds of games with one (mostly) shared skillset has a ton of appeal. Not to mention the assumption that games are either 2D or 3D is not always correct.
Unity3D is still the defacto for most gaming studios and I agree with the first point that the game engine space is very saturated. Still I admire all the work and great 2D engines can render faster on specific hardware (cocos2D was a good example)! I say focus on one niche and build better integrated tools for that niche.