The other argument is that the conventional meaning is different. Drag and drop is universally a "cut and paste" operation. If I drag text I expect it to re-position where dropped. Select-and-middle-click is a "copy and paste" operation.
It changes to "copy and paste" if you keep the Ctrl key pressed. The cursor changes and adds a plus sign to indicate this.
So it is flexible in that regard.
Unless the source is read-only. You can select text from the web and then drag-and-drop that selection into a text editor, and you won't change the web.
Unless you are using Gnome and the origin and destination of your drag and drop are on two different devices. In which case it will copy instead of cut.