requires creating a separate process? requires passing the memory from one process to the other.. requires a filesystem & requires the security analysis for having production software interact with the file-system?
One of the nicer things about using a separate process is that resource isolation is much better than in-process. You can kill the process without worrying about mutexes, OS scheduling applies priority consistently, the entire heap is freed once a job completes, there's no worry about leaking resources, and so on.
Even if you could do less I/O by keeping stuff in-process - and I'm not sure that's true if you build your graph in ffmpeg, it's just the source and sink of data which are external, and they'll almost always be external.
