Marco is also a professional wedding photograph if I am not mistaken. I think he actually knows what he is talking about. Generally the built in flash is too close to the camera, is obstructed by the lense and if direct facing.
Saying no one should use it is pretty extreme, but unless you have really poor lighting situation (nighttime outside or against the sunset), it's about 95% likely to produce a poorer image than without.
And this is advice for beginners who have no idea what a flash diffuser is.