I had hoped to write a follow-up to my post about an attention namespace for OPML before now, but I've had to stay on the sidelines of the discussion due to Thanksgiving, family and time spent on other work. Luckily, a number of folks have kept the discussion going:
As expected, I've heard a number of arguments against the idea of storing attention data in OPML, many of which aren't concerned with attention but instead with the suitability of OPML for any purpose. Quite honestly, these arguments seem moot to me given that OPML is already used by pretty much every aggregator. Sometimes the best choice is the one that's already being used even if it's not technically superior.
Rather than enter another format war, let's work to improve the situation instead. OPML is here to stay, and plenty of developers - including myself - will continue to use it and build upon it. Bickering about how OPML is technically inferior won't slow it down, any more than similar arguments stopped RSS from being adopted.
Another argument I've heard a few times is that storing attention data in OPML seems an odd choice given that Attention.xml was designed specifically for this purpose. Again, though, I'm coming at this from a practical standpoint, not a technical one.
To me, the point is that every aggregator already supports OPML import/export. Whenever someone tries a new aggregator, the first thing they do is import their OPML. Which is better: asking a user to import two files - one for their subscriptions, and one for their attention data - or a single OPML file which includes both the user's subscriptions and attention data? Your OPML subscription list already contains attention data in the form of the feeds you pay attention to, so it's not exactly a stretch to put more attention data in OPML.
FWIW, my preference would be to define an attention namespace for OPML which contains a subset of the same attributes that are already defined by Attention.xml. After all, the Attention.xml authors have already done the hard work of defining the important attention attributes, and with their permission, we should build on that instead of starting over (and as an added bonus, using the same attributes would simplify converting between the two formats).