It's actually really a cache to accelerate access to localstorage or a (future) cloud-based settings. The goal is to have a really fast user setting dictionary and not be afraid to use in big loops (it's used by the FrameRenderer object for instance in the preview frame list).
Perhaps it's a bit of pre-optimisation :) Either we rename to settings_/settingsCache_/... or we simply remove it.
Your call.