In a lot of games, 3D physics are just confusing and unnecessary. In an RTS, range detectors don't need to worry about height. In a top-down shooter, bullets don't need to worry about flying over an enemy. If these games could use 2D physics, their performance would be optimized by many-fold and it'd be so much easier for the developers. Currently, from what I've researched, Unity's 2D physics can't be used on the XZ plane which is what most 3D games are built upon. Thus, 2D physics often can't be used for top-down 3D games, or at least are very difficult to use. At the very minimum, the editor camera should be able to operate considering the Z-axis as vertical so a 3D game can be built on the XY plane. Other games that come to mind in which 2D physics on the XZ plane would be useful are Tower Defense, MOBA, RPG, and many more that have AI-controlled units. If there is a clean solution, please let me know. If not, make this essential feature happen by voting up the suggestion: http://feedback.unity3d.com/suggestions/extend-2d-physics-to-xz-plane.