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.

Good point on transactions and queries, but since I have to keep everything live in memory anyways...