I just tore through that book just a few days ago! Definitely a lot of eye-opening things happening in relation to rails and architecture.
I think a lot of people/projects/companies have been burned attempting to upgrade rails, or have a slow test suite, or having a terrible time trying to figure out where to fit core application logic into the rails stack.
And then you have an epiphany and say, wait, rails should not define my app. You say, I should be able to upgrade to rails 4 on day 1. Rails does not define my application, my application defines my application! So if you do just a wee bit of partitioning, it goes a very long way. (much to the chagrin of DHH, who for some reason evangelizes having your core logic in ActiveRecord models, to a fault)