From the docs, section 2.3:
MMA is designed to read and write files; it is not a filter.
Writing MIDI data to standard output is not practical, but it would be nice if MMA could read its input from standard input. I have some tools that generate MMA input data and it would be handy to pass the data directly to MMA instead of having to write temporary files.
Suggestion: If no input file is present, read standard input. -f option is required in this case.
Alternative: Allow standard input represented by a lone dash. -f option is required in this case.
Priority: Low.