This is why the Dog Brothers motto "if you see it taught, you see it fought" is nothing more than a clever way of saying "what we do is a reality-based martial art and not a fantasy-based martial art." That motto is true of all reality-based, and hence functional, martial arts. In contrast, when you see it trained and don't see it come out in fighting, "when you see it taught and don't see it fought", you know you have a fantasy-based martial art. And fantasy-based martial art isn't just a waste of training time, it is actually counter-productive, because you are training your body to move, act, behave, etc. in ways that will fail.
---But I have heard Marc Denny say that he still feels that a background in a "traditional" FMA is valuable. He has said that he values the "traditional" flow drills of Kali, even though you don't see them come out in fighting in exactly the same way as the drill. Just like Chi Sao, they help to build reflexes and responses that often come out in fighting in unexpected ways. Guro Denny is not an advocate of "throwing out the baby with the bathwater" and rejecting all traditional training. What he does is draw upon the traditional training to isolate and develop that things that work the best. He "functionalizes" the techniques, which is something we need to do with Wing Chun. But I get the impression that he would not agree with tossing out all the traditional training. Dog Brother Martial Arts is like the JKD of stickfighting. They have started from a "traditional" base and still draw upon "traditional" teachings in developing their own functional system.