Stderr output does not indicate that a program has failed. It's just a communication channel to the user that's not affected when you pipe or redirect stdout. So useful for programs where to want to make sure that data doesn't get mixed up with messages to the user.
exit codes are the only true way to tell if some process failed and programs should set them properly.
Exit status are inconsistent. Stderr output: is it a warning (ignorable) or an error?