If, and only if, a developer wants to target multiple devices. However, let's assume there was another Linux + Qt device available and the problems of Qt Components portability (i.e. arbitrarily stupid differences in import statements and APIs) were resolved.

If that device used hardfp & debian packaging we'd have binary compatibility between Harmattan apps and this hypothetical device's apps. That's a win-win scenario as those with either device can develop for both.

It's not free and it's not trivial and, as you say, it requires a willingness. Hobbyists develop for the devices they use, portability only happens if there's enough of a reason or no barrier.

There's no RPM version of Attitude for Mer-running devices, despite it being the first COBS app. It was fiddly enough trying to work out the best practices for using OBS without also targetting a different packaging format for a platform I'm not using.

Agreed. Speaking personally, that willingness to adapt and learn new stuff is directly correlated with my desire to scratch my own itches.