When you click on components with Shift key pressed, it adds the component if it isn't added yet, and it removes it from selection if it is. Rectangle selection with Shift key pressed works this way too, but I think it shouldn't. Let's imagine the next (quite common) situation: user selects an area by rectangle selection, but he forgets to add some components. He presses Shift and outlines forgotten components, but he probably will outline some of components already selected too. They will be removed from selection and it needs another one iteration which can create another mistake, etc. So I think Rectangle selection with Shift should be additional only.