In reply to several users: I'm not saying I agree with the cairn but the thinking was along the lines of the fact that:
There was already a marker there and most of the feedback they had was that one should remain as an important navigational feature. JMT have reduced the number of cairns and markers on the plateau to the minimum that seem necessary. They have spent a great deal of money and expended time and effort in scattering a large number of random cairns on the plateau.
A marker made out of natural materials is less visually intrusive and the small col the cairn is in means that its not really obvious/visible at all unless you are right upon it (and only those using mountaineering routes in the area will pass it rather than the majority heading for the summit).
The new marker is safer as an abseil anchor than the old one (which was just a post poorly planted in a small pile of rocks.