The point is no one will want to put all its datafiles on one volume defined by DB_CREATE_FILE_DEST for a production database.
The point is: simplify the work of the DBA. Let the mirroring and multiplexing with the system administrator on the OS level. "Why should a superior being like a DBA worry with those insignificant OS file issues?"

Anyway, OMF seems useless for people who likes a little organization and perfect for those lazy DBA's who don't even remember where they put their own datafiles. Sounds like crap.

As pando says, Oracle itself recommends OMF for test databases.