Hi Mark,
The only time I worry about my mast is when I look up, and it ain`t there anymore !
If you`re referring to the pink spinnaker boat, yeah I see what you mean, this sailor is easing his main & traveller quite a lot. Having said that he is one of the infamous top two sailors in our fleet, maybe he knows something ?
Most of us keep the mainsheet on, and play the traveller quite a bit. If we`re going very deep downwind we ease a lot of traveller, especially in light wind, or very heavy. If you look at the pic attached to this post, you`ll see what I mean. As long as the mainsheet is in quite tight, the traveller can be eased and the leech still acts as a backstay to a certain extent. An interesting point is that, since adding spinnakers, the only mast breakages I can recall have been sailing upwind, and the cause has been the same in two cases : the mast rotator device failed, causing the mast to over-rotate quickly with 2 sailors on the wire, and the masts broke just above the spreaders.
Our masts are about 9kg (extrusion only) so they`re not overbuilt by any standard.

Cheers
Steve