So what other reasons are there for a fixed framerate? Perhaps there are some, but I don't see "achieving perfect smoothness" as one of them.
Collision checking changes considerably at a rate higher than 60fps. This has already made me face situations where the character makes a "higher" jump in a very specific situation, which depended on the collision check with an object.
Despite that, at the moment I'm experiencing big problems in the physics behavior where the maximum FPS limit would be of great help. The framerate independent mode changes the strength of the object according to the processing, while the fixed mode, despite its equal path, changes its speed. Object speed can get very fast at high framerates.