well it's not only me who say it, Oracle says

Code:
Who Can Use Oracle-Managed Files?
• Low end databases

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1. A single initialization parameter, DB_CREATE_FILE_DEST, is 
specified. This is probably a low end database since all files are 
create in the same filesystem. You can seperate redo logs and 
controlfiles but not datafiles unless you create manually (so what's 
the point of using OMF if later, I have to create datafiles manually 
if I want to distribute I/O?)
OMF is for those shops with no DBAs, is for those peeps who does

create database and they have a database running, is for low-end servers for 1 or 2 or 3 filesystems assigned for Oracle, and only one of them is used for datafiles, no file distribution. In our databases we have file naming convention and if we see a datafile we know what tablespace it belongs, this is not the case with OMF, it's probably no harm but it's like SAP tables with dodgy names then if I want to know what is it I have to look up the dictionary table