My main motivation was for easily testing/debugging multiple configs (and for consolidating all the calls to QStandardPaths::WritableLocation so switching/migrating to the recommended AppDataLocation will be easier some time down the road). If others find it useful, so be it.
I'll consider changing it to SIGIL_PREFS_DIR dir before release. That will aesthetically match the "Open Preferences Location" button in the Preferences widget. I don't want to be the guy who can't take constructive criticism, afterall.