Yes, absolutely. There is no reason WebGL or WebSocket technology would prevent you from making a 3D MMOG client, or any game client for that matter.
Do you think future MMOs (and games generally) will wrriten with WebGL?
Yes. I believe within the next five years, most 3D browser games will be written using WebGL. The reason is simple - WebGL is the only standardized 3D technology that will have implementations available in every major web browser (Chrome 9, Firefox 4, Safari 6, and Internet Explorer via Chrome Frame).
Let's say your development team was you as a developer, and another model creator (artist). Would you use a library like SceneJS for the game, or write straight WebGL? If you would use a library, but not SceneJS, please specify which.
Use a library to save time. There is no reason to write your own WebGL graphics code unless an existing library is missing features that you need. Even in that case, it would probably be more time efficient to extend the existing library.
For my project, I am using GLGE as it supports many different graphics effects and is constantly being updated with new ones.