This is one of the worst things about the XMLs. If someone says "unreadable!" (and (s)he is right!) the apologists answer: "it's meant to be machine-readable, not human readable". If someone says "redundant!" (WTF do you have to repeat the whole tag name on close? Why are there *two* quote characters for attribute values? Why the F don't you have a plain straight regular escape syntax in attribute values, but have to mis-use entity syntax for that (entities are meant for completely different things, remember!), creating the mess with "internal" vs. "external" entities? On and on!) -- then the apologists retort: "it's to catch (human) input errors!".
I have another theory: XML is a denial-of-service attack on all of us.
OK, to be more precise: XML is passable as a (traditional) document representation language (family). As a data representation language it's plain perverse.
And yes, the line between document and data is somewhat broad, but for maths (and music, and vectr graphics), I'd say we're clear off this line... firmly on the "data" side.