Comment by ConstantineXVI

The difference is this one plugin is capable of covering much more ground than Unity3D alone. Once it's there, nothing stopping one from porting, say, id Tech, Unreal, Frostbite, RenderWare, OGRE, Avalanche, Crystal, CryEngine, EGO, Source, etc. to NaCl and running it in the same plugin. Say what you will about Google's strategy, it opens up quite a few possibilites.