cache_form is never cleared on some sites

As per title, the form retrieved in cache_get would never invoke db_query("DELETE FROM {cache_form} WHERE expire != CACHE_PERMANENT and EXPIRE < time()", ) because cache_flush_cache_form will always be 0, manually setting this to a value would only invoke the cache cleaning once.
I propose adding variable_set("cache_flush_cache_form") to form_cache_get, expiry is already set to a long TTL, so this should be safe to clear the cache, instead of using a cron to regularly truncate the table