You are right - this is not a method promoted by anyone other than myself but I would like to show that there are options for eventers that need improved traction. And if they don't - great! I know many horses that don't need anything, but my horse, and some clydie cross types, benefit from some tread during the XC or when sj on grass. I have experimented over the past 5 years, and where I was initially taught to do heel notches (imitating the action of studs) I have found the toe tread more useful in the slick clay conditions we often encounter.
The example you see here is very extreme, for a 3DE in wet conditions on clay (mid winter 2009). However technically it is just outer wall allowed to grow to ground level and then notched, so not as dramatic as it looks.
Just putting my one rat study out there to see if there are any others.