Initially scoped attributes were able to override other attributes with the same name if they were already registered by the time when a scoped attribute encountered. This allowed some interesting use cases like this:

BOOST_LOG_DECLARE_GLOBAL_LOGGER(my_logger, src::logger_mt)

void foo()
{
    // This scoped attribute would temporarily replace the existing tag
    BOOST_LOG_SCOPED_THREAD_TAG("Section", std::string, "In foo");

    // This log record will have a "Section" attribute with value "In foo"
    BOOST_LOG(get_my_logger()) << "We're in foo section";
}

int main(int, char*[])
{
    BOOST_LOG_SCOPED_THREAD_TAG("Section", std::string, "In main");

    // This log record will have a "Section" attribute with value "In main"
    BOOST_LOG(get_my_logger()) << "We're in main section";

    foo();

    // This log record will have a "Section" attribute with value "In main" again
    BOOST_LOG(get_my_logger()) << "We're in main section again";

    return 0;
}

However, this feature introduced a number of safety problems, including thread safety issues, that could be difficult to track down. For example, it was no longer safe to use logger-wide scoped attributes on the same logger from different threads, because the resulting attribute would be undefined:

BOOST_LOG_DECLARE_GLOBAL_LOGGER(my_logger, src::logger_mt)

void thread1()
{
    BOOST_LOG_SCOPED_LOGGER_TAG(get_my_logger(), "Tag", std::string, "thread1");
    BOOST_LOG(get_my_logger()) << "We're in thread1";
}

void thread2()
{
    BOOST_LOG_SCOPED_LOGGER_TAG(get_my_logger(), "Tag", int, 10);
    BOOST_LOG(get_my_logger()) << "We're in thread2";
}

int main(int, char*[])
{
    BOOST_LOG_SCOPED_LOGGER_TAG(get_my_logger(), "Tag", double, -2.2);

    BOOST_LOG(get_my_logger()) << "We're in main";

    boost::thread t1(&thread1);
    boost::thread t2(&thread2);

    t1.join();
    t2.join();

    // Which "Tag" is registered here?
    BOOST_LOG(get_my_logger()) << "We're in main again";

    return 0;
}

There were other issues, like having an attribute set iterator that points to one attribute object, then suddenly without seemingly modifying it it becomes pointing to a different attribute object (of, possibly, a different type). Such behavior could lead to tricky failures that would be difficult to investigate. Therefore this feature was eventually dropped, which simplified the scoped attributes implementation significantly.