That's a pure efficiency perspective. When discussing content sources, however, privacy is also an important concern. By necessity, a push-based protocol needs

That's a pure efficiency perspective. When discussing content sources, however, privacy is also an important concern. By necessity, a push-based protocol needs to know who to push to. A pull-based protocol only needs to respond to requests, which may be anonymized. The operator of an RSS feed gets a list of source IP addresses of feed requests - and that's it.