I agree with the principle, but I'm not sure how to achieve it in this particular case.
The issue is that open badges need to be awardable across courses, or for things unrelated to any course, so an activity module doesn't seem appropriate since you don't have open badge "instances" which belong to a course. We do intend to create a block for some of the functionality but again it doesn't seem appropriate for everything.
The approach we're taking (which seems to be the way core features like completion, blog or grade do it) is to try to keep the code as separate as possible (within it's own directory and library file), but still use lib/db/ for code upgrades etc.
Personally I wish that core features were more isolated from each other - if there was a "core" plugin type with each feature having it's own "core/[componentname]/db/ directory" rather than having it all together in lib/db/ it would result in less conflicts when merging in changes.
Given that's not how it's done at the moment are there any other plugin types which are appropriate for a feature that's acting at the site level?