Well, it's interesting. My example is partly contrived and partly not contrived. I worked on a big application where the web front end was very much the tip of the iceberg. We had the Rails monolith problems that you frequently hear decried. One thing we at least thought we would really like to do was move to an architecture with a bunch of little services talking to each other. We thought about having them talk to each other over tcp or 0mq, and we thought it would have been really nice if it were easy to take our existing application logic and put tcp or 0mq adapters on top of it. I keep saying "we thought", because we never actually did any of that, which may have been the case either because our application logic was so tied up with Rails that it was impractical to pull it apart (as some of us thought), or because it's a silly idea to begin with.

We did have a good deal of success pulling out libraries though, and separating concerns helps with that too. (So maybe I chose the wrong argument to make in my comment before.)