I'm not aware of any Unix system where cron sends mail for jobs based solely on the exit status. I believe AIX cron includes the exit status in the subject when there is output.
I can understand why you might like to be notified by jobs that have a non-zero exit status, but I think this is too big a change in behavior to be the default. I wouldn't object to a crontab setting similar to MAILTO that can enable the behavior. - todd
