If this was the only criterion used, it would be pretty stupid.
As it is, it is one of many.
And thus it has its use. Apparently the editors for some time just did base it on course rating/slope, but in recent years have decided to add a little subjectivity and let the raters chime in. Who knows which works better, we'll see.
But in any case, I'd like to hear from Rich Goodale on this, who said that Carnoustie is the greatest course on the planet, because, in so many words, it is great for one and all AND can host championships (ie is tough enough to test the big boys). Matt Ward used the same standard for trumpeting Shinnecock Hills as the greatest. I believe that's what GD has in mind in using this criterion... that for absolute greatness, a certain raw difficulty helps.
And since it is one of the many criteria measured, well... a course that's off the charts on this criterion - that is outrageously difficult - isn't necessarily going to get a high overall rating, because all of the other criteria need be satisfied also... and on the other side, a course that doesn't have this raw difficulty can make up for this by being very strong in other areas...
I see no harm in keeping it in. Just don't value it any more than the other criteria, that's all.
And GD only does US courses anyway, so this is a moot point for Royal Melbourne!
TH