>> A Properties file can be as simple as set of key,value pairs like an INI file or an XML dump of object attributes or whatever, just basically storing data as a flat file <<

I find that collections often later need more features than raw flat files can provide. I don't like to change API's just to get the next step up in features.

(Note that some ODBC drivers can read/write to flat files. The physical representation is just another attribute.......in theory.)