Although Rails code can be straightforward if you stick to the conventions, many real-world apps call for something more than that. So in practice, it's usually necessary to perform at least some level of metaprogramming, or at least some more exotic configuration.

Also, if you're using Rails library, you'll notice the hipster aspect of the Ruby/Rails community, which is to say many libraries are active for 1 or 2 years before the developer gets bored and someone else comes up with a hotter alternative. That's cool, it's quick evolution, but it also means real-world Rails developers have to spend a fair amount of time dealing with libraries that are either no longer maintained or are not yet mature.