Think about it this way: would you use a string to represent a date? You could, right? Just create a string, let’s say “2012-06-25” and you’ve got a date! Well, no, not really – it’s a string. It doesn’t have semantics of a date, it’s missing a lot of useful methods that are available in an instance of Date class. You should definitely use Date class and that’s probably obvious for everybody. This is exactly what Primitive Obsession smell is about.
Rails developers can fall into another kind of obsession: framework obsession. Rails gives you folders for models, views, controllers, etc. Everything has to be one of those. Logic is shoehorned into models instead of put in objects unrelated to persistence. Controller methods and helpers grow huge with conditionals and accreted behavior.
This is partially an education and advocacy problem. Luckily, folks like Avdi Grimm, Corey Haines, Gary Bernhardt, and Steve Klabnik, amongst others, are spreading the word of how to use object oriented principles to design Rails applications without obsessing over the constructs in the Rails framework.
The second part is practice. Once you’ve educated yourself and bought into the notion that a Rails app isn’t all Rails classes, you’ve got to practice and struggle with the concepts. It won’t be pretty the first time; at least, it wasn’t for me. But with time, I’ve come to feel far better about how I design applications using both Rails principles and object-oriented principles.