Mick, I also work for people but I care for trees. After a lifetime of doing so, trees have earned my respect. They are marvelous organisms and I will always advocate for their well being.
When pushed by circumstances, I will cut way more than is good for them if it means they get to live. A live tree has a chance.
Dead wood removal in young trees seems to be the most beneficial. Whether in the form of natural senescence, disease, or mechanical injury, cutting it out will just let the tree get on with growing.
Mature trees, especially those that have not been worked on for decades, are much different. Because trees grow and develop in direct response to their circumstances, dead wood that has been there a long time can be surrounded by growth that was influenced by its presence. As several people have already pointed out, removing dead wood from those trees can have strongly negative health effects.