Without it, publishing would be stuck in a morass of nebulous, ill-documented proprietary messes, and a great deal of current learning would be at risk of being lost to posterity. The fact that there are associated open standards such as XSLT with which to transform it is just the icing on the cake as far as publishing is concerned.
This is why there's so much distaste for XML - people try to use it for applications where it isn't ideal (and there are many more of those than there are applications where it is ideal) because they've swallowed someone else's hype, and as a consequence they have a bad time. If not for the unbelievable exaggeration a few years back (I heard people claim without irony that XML - a markup language for god's sake - would literally change the world), the divisiveness wouldn't exist, and it would be a technology used by experts quietly getting on with the jobs it's best for.