I don't agree with this argument. You can make the same argument against cat >cat access.log | head -n 500 | grep mail | perl -e … >we find that cat performs

I don't agree with this argument. You can make the same argument against cat >cat access.log | head -n 500 | grep mail | perl -e … >we find that cat performs two responsibilities: >1. Printing an error to stderr if the file doesn't exist >2. Copying a file to stdout