You are only excluding 'user' messages. I don't know the list of which msg types are 'user' messages off the top of my head, but it isn't that long. I don't believe that crond sends any other user messages (but it wouldn't be the first time I was wrong). You would still audit things like AVC denials for cron or or any syscall audit rules you have. Basically that is going to deny all audit messages that cron explicitly sent to the audit system, but not messages generated by the kernel for cron.
I can't really answer whether this is good or not then, as 1) my auditd knowledge is still limited and 2) I do not really know what these "user messages" actually cover (is there a definite list of these?). I would like to disable the following types for sure: USER_ACCT, CRED_ACQ, USER_START, CRED_DISP and USER_END.