The advantage of returning a rAtom object here is that the atom representation is then composable, for example if you have a Blog class that has many posts you can do this:
To get this atom representation out via HTTP, you would do this in the controller:
This has been really useful in an application that I am working on because we are using Atom as the main communication format between a bunch of different components, so this composability comes in really handy. For a simple blog it might be overkill, but at least you get the much better performance of libxml-ruby.
You'll probably notice a bit of ugliness in the to_atom methods, specifically this:
The URL is hard-coded here because url_for is not available in a model. This is a bit of trade-off between the composability and strict MVC here, if you really wanted to you could pass the URL in as a parameter, or add a block that generates URLs or whatever, it's up to you.
For some applications, like a simple blog, the composability might not be too important, and you might prefer the syntax of atom_feed in Rails, or at least want to be able to render the atom in a view. In this case it would probably be nice to have a rAtom based template format, so you could add a view like "blogs/show.atom.ratom" which allowed you to move the to_atom method into a view and call url_for directly from the view. I'd like to add this at some stage, but it hasn't happened yet, due to time and priorities, but if anyone would like to submit a patch for it I'd gladly accept it.