Note that the examples you used (MS Office and Adobe applications) are big programs and have lots of features. Users will be dealing with that program, and only that program for much of the program's lifetime.
Newer versions of MS Office (2007) and Adobe Photoshop (CS4) use multiple windows and tabs, respectively.
Note that with Windows 7, MDI's will probably lose popularity even more because of the extra power of tabs given by Microsoft's API's (although you needn't strictly use tabs -- MDI windows could work, but would be more confusing for the user than usual).
