Not even just an application. The whole phone needs to be consistent. Every app in it. Otherwise it's just repeatedly discovering and forgetting how to use each and every app's unique interpretation of what their average user wants and what their average user thinks is intuitive.
The biggest reason why consistency is more important than following platform guidelines is for cross-platform apps available on multiple devices that have different platform design guidelines. It's obviously not feasible (and I would argue not user-friendly either) to switch the order of action controls for the same app on different devices, especially when the app is available via a web interface as well as native.