I understand the point about the discussion regarding site-specific namespaces being open for almost a year, but the point is that I didn't read it as I don't actively monitor it, and I'm quite sure that many other developers didn't read the discussion either.
Kghbln suggested that the discussion about site-specific namespaces should be raised on the appropriate mailing lists, which in this case would've been wikitech-l, the development mailing list (yes, mailing lists are so Web 1.0 but they're also how we do things). Many developers, whether they're core, extension or third-party, are subscribed to that mailing list, so by posting there your message will reach the intended audience and you can claim consensus on your changes.
As for the question regarding the purpose of this page, it really depends. To be exact, while many developers might use this page, it still doesn't change what's in SVN — for the time being, BlogPage is using namespaces 500 and 501, no matter what this page says.
Ignoring everything and anything related to site-specific namespaces and namespaces indexes 500-599, what about the other namespaces? While Wikia certainly has a lot of custom extensions and some of them do define their own namespaces, I feel that registration should happen on a per-extension basis instead of reserving a range to Wikia; are their developers even aware of the fact that they had such a reserved range registered on this page?
Extension:NagiosConfig, which has reserved namespaces 600-609, is marked unstable and the latest edit to the extension page has been done on 27 May 2011 and its source code isn't available; Extension:FanBoxes is older, it is marked as stable and it certainly works with MediaWiki 1.16.0, so I feel it should get the namespaces 600 and 601, as it already uses them in the code.
Considering that Extension:TimedMediaHandler hasn't been deployed yet on Wikimedia wikis, I don't think bumping its namespaces from 700 and 701 to something like 710 and 711 or something will be very controversial.
This post was posted by Peachey88 (Flood), but signed as Jack Phoenix.