As it was said in a discussion here in HN a couple of says ago ("Welcome to the new web"), RSS and Atom feeds are becoming less and less supported. Which is a sad and bad thing with regard to the internet of things that is to come with Twine and GreenGoose and Arduino/xbee/sensors.
The appropriate protocols are there. Please use them.
pubsubhubbub doesn't have privacy/authentication/authorization mechanisms as part of the protocol, so creating near real-time push APIs isn't just as simple as using some pubsubhubbub library. I think its the complexity of coming up with one's own auth system that stops a lot of people from using pubsubhubbub. However, it is pretty awesome for things that are published to everyone.