I am not so sure with that. Say, your program is used in a shell script and is invoked badly - you might want to print its usage then. If you exit normally your shell script might break weirdly but if you exit with error it's easier to spot the reason of failure.

On the other hand you made me thinking and probably you should have three code passes per default:

  [0] normal behaviour (exit 0)
  [1] bad arguments (exit EINVAL)
  [2] --usage (print to stdout but but exit != 0)?

Anyway I am not sure if it makes sense to declare "usage" as normal behaviour.