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.

>>5
Concurrency without locking being a pain in the ass doesn't convince you? Well, your queries must be simple then. For now.

A RDBMS lets you expand your schema and use-cases far more easily than an in-memory setup would. Also if the application process crashes, state is recoverable from no later than the last time a transaction was committed.

Though for a non-networked desktop application, relational databases are WAY OVERKILL. Don't be the guy that writes an application and depends on a relational database for the app's config file.