How make convincing case to DBA to use Oracle external tables for 500 MB to 5 GB files?
IMO, an external table is much easier to manage and flexible than a SQLLoader script.  So if you're already doing a recurring SQLLoader load, I see no possibilities of evil in switching to external tables.

By doing the external table, even though the base of the table resides in a text file outside the database, the data can be accessed from within the database, no need for a separate SQLLoader tool/scripts.  So you can write procedures/packages to manipulate the data straight from the external table.  No need to load it anywhere...as long as it's in the directory you have set, it's already "loaded".

As a DBA, I'd much prefer managing a DML script using an external table than having to manage a SQLLoader script.  But SQLLoader has been around a while....if your DBA has too, it may be a tough sell. :)