To me, the intricate pile of MUSTs and SHOULDs piling up in Atom format spec is a display of short-sighted technocrat bullshit.
For example, what is the point of requiring all entries to have a unique identifier when feed clients have no incentives to reject feeds that violate that requirement? Filtering of duplicate entries from multiple sources is a quality of service problem that is best dealt with in a separate opt-in specification. Clients and servers that support the opt-in spec will be more popular if resulting differences in user experience and distribution are significant enough. If not, there was no need for it anyway.
Ignored mandates are much worse than ignored opt-ins because mandates leave no room for failures, competitions, nor changing needs. Opt-in approach can also embrace RSS so that clients can check for the extension elements regardless of what the container feed format is.
Will the Atom WG see the light? I doubt it because they are thinking more about their product than the market.