I'm not the author, so take what I say with a grain of salt.
I don't think that beginners will be able to benefit from the book as much as the book deserves, so to speak.
Basically, if you have written a few rails applications, you will recognize several things:
1. You see the anti-pattern, and recognize something you did (along with the reasons you chose to do it)
2. You understand the discussion of why it's not really a good idea afterall
3. You get the benefit of a nice a-ha moment when you see how to solve the problem in a different way
If you don't have any experience with rails, there's a chance that much of the discussion will just be words.
Said another way: if you've made the mistakes, the pain you felt making the mistakes will make it that much more efficient to learn from the book.
If you mean practically, then yes, these anti-patterns have been picked up from live/real world rails implementations that have gone wrong.
As RoR is maturing as a platform, we need to be aware of the common mistakes that occur in it (by java, php or .net developers) and have a roadmap for refactoring our existing codebase.
Chad and I have had a lot of exposure to "interesting" rails applications as consultants (Thoughtbot, and independently). These AntiPatterns are definitely issues we've actually seen - no armchair philosophy coding, here
subject: Question about the RoR anti patterns book