Sure it's obsolete, who needs databases and live, chancing data. All we need is a static pages. Besides who needs to build his own infrastructure, it's 2012 right ? Let's buy it.
I think for the average use case the answer is no, nginx doesn't buy you much. However nginx is a lot more flexible than cloudfront, so if you have more complicated caching rules and such nginx is a perfect fit.
Don't get me wrong : cloudfront is a cool tech and obviously useful too but a bold claim like " we don't need nginx ( or any performance oriented web server ) because we have cloudfront " is just the very wrong way of thinking.
If you need to build a toaster, you don't need to build an iron smelting plant. Certain things other folks are better at taking care of.
Looks like we need a sarcasm alert on HN, like spoiler alerts :)