I find that the principle of marking tasks based on urgency/importance is not useful after all.
When the workload is manageable, it is most possible to prioritise based on importance and urgency. However, people tend not to do that since it is already manageable.
On the other hand when the workload starts to get more than you can manage, it is even harder to label your tasks which are not considered urgent and important.
The worst thing for overloaded work to happen is to let certain tasks fail because of your prioritisation. This is not acceptable either. You have to look for another solution to tackle them so as to achieve win-win situation such as outsourcing and technology leverage. Another simple way is to rework the timing so as to have ample time to achieve them all.
Image Credit: Martin Terber