They are largely used, and appear as mixed blooded/breed creatures rather than humanoid dragons. It is very rare that you get a humanoid dragon, rather; you more often see draconic sorcerers who have an innate magical sense/abilities due to their dragon blood. The depicted dragon has always been odd to me as a half dragon is more of a monster's attribute than a characteristic. They come from the monster manual, but can bee applied to characters. I have only seen practical application as mixed blood. On one occasion I fought a half dragon, like the depicted image, but he was of magical creation with basic monster level intelligence meaning he really couldn't lead a normal life like a human.