If you are in the Finder, the "cut" option is disabled so because otherwise if you actually selected "cut", whatever file or application was selected would be removed from the window/desktop, and placed in the clipboard. That wouldn't be a good idea, as if you then cut or copied something else, the first item would be deleted for good.
However, if the selection in the Finder is the text of the name of a file or application, then you can "cut", as that's just affecting the name and not the actual file.
I hope I've been clear enough.
I guess Apple decided that allowing people to "cut" files and application's would be too risky. I tend to agree with them, if you want to trash something, there are better ways.