Fully agreed, though the exact proportion may or may not be applicable for the particular position. I've progressed in my career by doing exactly this and see how it has benefited me relative to those who don't do it (or just aren't interested in it). I've been lucky to have managers that have supported this by checking me when I've taken on too much and urged me to assign tasks to more junior team members (which in itself turned from the "other" work into my main work). It results in a healthy team where all those who are willing have the opportunity to grow and expand their skillsets.