Yes, it would make sense to include emissive channel in the default shaders (it's quite a basic feature), if it can be done without exploding the pixel shader permutation count. Possibly needs to be put only to the ambient permutations and avoid the per-pixel (dir, spot, point) permutations.
However note that the texture units and uniforms include stuff that simply isn't addressed by the default shaders, but they're just acting as placeholders allowing you to implement stuff. And of course there will always be some effect you want that is not included in the defaults. It's same with something like Ogre, you should be prepared to customize your own shaders / techniques / materials.