Believability of a fantasy creature can go so far. Case in point--the title feature. I have first seen it on a Ringwraith's winged mount... ...then on Smaug... ...and finally on the dragons of Tui T. Sutherland's Wings of Fire series. I never understood the appeal. Bats don't have it. Pterosaurs certainly didn't have it. It doesn't add to the wings' appeal--it just makes them look scrawnier. Without the need for a spike growing from its elbows, a bat's wings look better and cover more surface area needed for flight. So in regards to adaptation, what point would elbow spikes on a creature's wings serve?