That is very naive. Rails takes care of model persistence. But what happens when you need to join a ton of data... Sure you can fetch all the records and transform on the web server, or you can ask your database to do it in a fraction of a fraction of the time it would take ruby to do this.

Example:

Select all users who have an entry in their shopping cart with an order greater than $200 and tally up how many users there are by state.

Databases can usually do this with a few index lookups and give you results in seconds from hundreds of millions of records. There are no objects to map, just data and math. Then boom you get like 50 rows returned with a state and a tally. Try selecting all those tables building orm objects and transforming that data on your rails app.

And that's always been my take at it.