I'm completely on the opposite end of the spectrum: engine dummy apps aren't just great for running specs, they're great for running in dev mode (especially whe

I'm completely on the opposite end of the spectrum: engine dummy apps aren't just great for running specs, they're great for running in dev mode (especially when the entire integrated app is a huge slow beast), and for exposing dependencies on the main app, which you ideally want to eliminate. Essentially, the more difficult it is to get your engine running with a plain vanilla dummy app - the more you have to pull in or mock out from the main app - the more tightly coupled they are, and in a gross circular dependency relationship too. If you want the benefits of modularity and separation of concerns, you want the relationship between the engine and the main app as simple as possible; keep the API small and well-defined, and the dependencies going in a single direction.