The whole "dummy app just for booting specs" is silly, though. I ran into a ton of trouble trying to get it to play nice, just because my Engine had some dependencies from the main app. Not to mention fighting with rspec.
I agree with you if it's intended to be packaged as a gem, but if I'm only using it as a namespace separator, then it's pointless to fret about where it all goes - they were never meant to be used separately in the first place.
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.
I agree with you if it's intended to be packaged as a gem, but if I'm only using it as a namespace separator, then it's pointless to fret about where it all goes - they were never meant to be used separately in the first place.