Jeremy Ashkenas hates cross-browser consistency
github.comUnderscore is not a kitchen-sink library. In order to keep it a terse library, Jeremy will need to constantly decide what goes in and what does not. I don't always agree with his choices, but I'm glad he's willing to be aggressive about what he believes is good. That's what makes for good libraries.
Also, I must say this is a poor choice of title for a HN post. It's emotive, a generalisation and doesn't actually reflect the actual content of the link.
You obviously disagree with Jeremy. Fine. But posts like this are weak.
If what you want is cross browser consistency of sparse arrays (that is, if you _really_ want to use them), then you should probably use ES5-shim or something instead.
Jeremy Ashkenas can support or not support whichever parts of built-in array behaviour he wants, given that:
a) It's his library b) It doesn't try and copy the standard ES5 API anyway