Why can't you set an element's value?

I'm trying to be patient and learn the reasoning behind some DOM design 
decisions.  To that end, let me ask: Why does setNodeValue on an Element do 
nothing?  Given that XML is all text, shouldn't every element be able to 
contain text as its value?  What other kind of value would it contain, 
anyway?  Why do we have to add a text node?

There is question after question on various DOM-implementation discussion 
forums from people who are baffled by the fact that setNodeValue does 
nothing, so I know this is a source of bewilderment for many.

Regards,
Gavin