SysAdmin's have a definate love/hate relationship with RMAN. Many of us not so jokingly refer to it as the "Recovery Mangler". You love it because its fairly simple and is the only sure-fire way to perform online backups. You hate it because it's often cryptic, and either works or doesn't. Because RMAN couples closely with a backup infastructure there is alot of finger pointing when problems arise between "It's an RMAN issue!" and "No, it's a NetBackup issue!". RMAN causes DBAs and UNIX admins to get too close for comfort and niether generally understands the wholistic view of the interaction properly.
So, why would a sysadmin like RMAN?
The problem with backing up Oracle using traditional methods is similar to the problems with backing up filesystems, unless you shutdown the database and perform a cold backup there is no way to know that all the transactions and changes have been written to datafiles. The SGA maintans a huge amount of data in active memory which can cause a problem. It's alittle like editing a configuration file on the system and then halting the system and wondering where your changes went. In order to ensure consistance of the database we need a hot backup method. If we restore a filesystem backup of the database that was taken while Oracle was running we run the risk of lossing database changes at best or having a corrupt database at worst.