Based on the description, if mappingsonly is true, the cache shouldn't work if the admin hasn't explicitly set a cache store for the cache.

To me, this would be helpful if you are doing something that should be faster fetching from the DB over the default file based store, but you want to allow admins to switch it to something very fast/low load like memcached.

When mappingsonly it set, cache_config->get_stores_for_definition() properly returns no stores, but cache_helper->get_stores_suitable_for_definition() assumes that the lack for stores is unintentional (the assigned caches are unavailable) and adds the default store back in.