In my current position, our company uses tivoli software to backup our servers nightly. Files backed up fromt he database servers are kept on the backup server for 30 days. Our RMAN scripts are set to backup the databases nightly and delete the second oldest rman backup on disk after the backup is complete. That is, we keep the last 2 RMAN backups on the database server knowing that the 2nd oldest RMAN backup has already been written to the backup server. So what we have on the database server is a backup to the backup. 🙂 What we are counting on is that should the server have a problem and the current backups crashes we at least have the previous days backup on the database server disk which would allow for quicker recovery. Of course we tend to believe there will never be a problem with the backup on the disk but at least we have the other backups stored on the backup server we can retrieve if necessary. Even then we create nightly tapes from the backup server that are stored off-site that we rotate on a yearly basis.
Some companies think that you should keep a week’s worth of database backups on the database server but this could lead to excessive disk space usage. Your mileage may vary.