Nice post Surapon.
One time I was out shooting and sadly, my flash died. I was glad that my 50D had a pop up to catch a Dik Dik in the shadows, which I otherwise would have missed.
Sorry, I am not the PRO, But some of my Canon DSLR have Pop-Up Flash---Yes, It most use full Fill - in light when The sun at the back of subject---BUT I must use my DIY White paper as the photos below, or Use my Clean tissue in my pant pocket cover the Pop-Up flash and tie down with rubber band.
Yes, Cheap and work for me , But for Fill flash/ Pop up flash with white paper cover must to be 4-5 feet distant from the Lens to the face of portrait model----Yes, Instead of cheap white copy paper, We can use the Wax Paper or Translusive Drafting Paper too= Yes It work better than White Copy Paper at the longer distant from the subject.
Yes, I have a great Boss/ Great Architect ( for 5 Years during 1975 to 1980),who Teach me = " The Great Architects same as the Great Chef , who can use the Chicken's S--t to make a great Chicken Salads"---Yes, Some time Useless Pop-up flash can be the Life saving Photos for the photographers like us, if we know how to use them.