What will a RDBMS give me?

I'm writing a little application that keeps all its data in memory. Modifications are done through an XML/HTTP API. When a modification happens, I change the in-memory structures and save the record to the DB.

What do I lose if I just serialize my in-memory objects directly to individual files with their name? Say "myconfig/users/1", "myconfig/accounts/1"? I save out on having to use an OR mapper to get things into RAM as well as having to specify the exact schema up front.