The Common Access Specification that IBM developed in the middle of the 80ies, and Microsoft championed for many years in DOS programs and in all versions of Windows, has become the natural way of interacting with most programs. You know that the Copy and Paste commands will be under an Edit menu, so there is one less thing you have to learn for a new program.
CUA was and still is very important. For example,I remember the discussions on whether or not the guidelines had to be followed (for example, the early Turbo C++ gray "chiseled style" dialog boxes were not standard, as dialogs had to have a white background, and this was subject of debate. Eventually, Microsoft changed the specs and moved to gray background for dialogs). As another example, Delphi has a menu template providing a CUA-complaint menu structure as a starting point for your program.
Now, the fact is that the forthcoming Microsoft Office 2007 breaks the CUA. You cannot find Paste under Edit, because menus are gone! I shortly tried using the Ribbon, and found it a little frustrating, as I had to spend time finding even trivial commands.
This is explained in much more detail by Alan Zeichick in this SD Times article, and also in his blog. Most of the operating systems (beside the Mac) and applications are CUA-complaint. The web is an exception as it uses a different metaphor (I personally hate pull-down menus in web sites). Other exceptions are mp3 players and video player programs, which tend to embrace the web model, and still most of them are more CUA-complaint than the new Office.
As Zeichick says, "Microsoft has broken the GUI model that has served it so successfully for two decades. This is a big mistake... It will also make the CUA-compliant alternatives, like OpenOffice, more attractive." I agree.