I did a search online and didn't find any use of
fluid_cmd_handler_register() except where FluidSynth was included in other projects and API documentation. So at least from an initial look it seems like this function is not really used by other projects.
I think I can see how this could be used though. This could allow for an application to add additional commands to the shell command set, assuming the default command handler instance can be retrieved (didn't check that). Might be useful for an application to extend the default shell commands. I leave it up to you though as to whether you think that is useful or should be handled in another way which doesn't expose the fluid_cmd_t structure.
Best regards,
Element