In Mac os x would it not make sense to allow not currently open documents to come onto the dock? Why is this not part of the UX?
I may not be fully understanding the question; so, I will try to answer with my interpretations of it.
- You can place a file in the Dock w/o opening (and minimizing) it - between the application portion of the Dock and the trash can. Just pull the file from Finder onto the Dock like you would anything else. (Did this with an image while writing this answer and Preview did not launch until I clicked on the image, in the Dock.)
- The dock is, primarily for applications and/or directories, not individual files and the main UX is in place to support this - all applications are on one side of the dividing line between the application icons and the trash (directory).
- You can add a directory housing documents you decide through your personal IA preferences to place in that directory onto the Dock, which can then display the contents to you in a variety of ways - the UX for this is pretty nice in my opinion. (I do this for my documents directory now.)
- In Pages (not sure if this is something other applications can take advantage of), if you move into Exposé for the application, it will show your currently open and recent (unopened) documents under the active window; so, the functionality is there - just not permanently on the Dock - per point 2.
In a nutshell, you can add individual, unopened files to the Dock - the same way we add just about anything to the Dock - and remove it the same way. However, the primary purpose of the Dock is for Applications and Directories (the Trash being a directory itself) - http://support.apple.com/kb/ht2474 - not individual files (the Dock would/could get very cluttered, very quickly - not good UX).