Now databases are not 30 or 40 or 50 gigs .Every Major database is 200 or 500 or in Terabytes ...

Rman does not copy the whole datafile .It copy's the datafile block by block and the empty blocks are not copied .Hear you save Time and the Tape space .This is one main Avantage .

The catalog database is having all the details about the backups .First few days till one is familiar RMAN will be a trouble (It generates of course lots of errors .that to when you have to relink the oracle with the media manager ...
Once it is done ....Rman is SMOOTH ....