The point is that you can't "pipe GUI output to other command", the GUI would actually have to serialize things in a useful way and send to that fifo. Similarly you can't send stuff to it's stdin and expect it to do anything sane.

Further, since you can't seek() in a fifo, a lot of likely GUI applications involving files would break on trying to deal with a fifo. Also the typical GUI app on read doesn't assume a 'tail -f' like approach to arbitrary file inputs.