>In the form of the Kate codec, which supersets SRT as far as I know...
On a theoretical level, yes, on actual technical level, no. In that aspect it's about on par with SRT, except a lot less supported.
>... and "mature support for encumbered codecs" is most of your feature list.
For your average user backing up their DVDs or encoding their shortfilms or doing whatever it is quite important, since more often than not these people want to get the best quality they can get, in which case for video they turn to x264, the best video encoder in the world.
My main argument against Ogg in the web is that it separates content to web and non-web. I'd rather have Theora+Vorbis in MKV than Theora+Vorbis in Ogg, though more preferrably I'd use H.264+Vorbis in MKV for my web video needs if HTML5 <video> supported that properly in major browsers. I'm quite sure many people would be delighted about this too, since it'd basically mean that their personal copy and the web copy can be the exact same file without compromising quality.
>I've never seen an MKV file using chapters myself
And I've seen thousands and made hundreds. Quite literally. And I gotta say that I love chapters. I backup plenty of TV series I own and with chapter support I can easily skip for example the opening and ending themes of a show. With segment linking, I can even have the opening and ending in separate files and seamlessly link them to the episode files, which saves space.
In general I hate how ignorant the whole media industry seems to be of subtitles. Subtitles in DVDs look atrocious, even Blu-ray subtitles are more limited than they could be, MP4's subtitle support is a joke... it seems like no-one in the industry cares at all, which isn't a surprise considering it's mostly based in the US where subtitles are a rarity. Around my part of the world everything that's not for very little kids and is in a foreign language is subtitled. This is the case in quite many countries, and for people living in those countries, subtitles matter as well. Subtitle support needs to be taken more seriously!
How about you Xiph guys rather spend your time trying to kill software patents completely? I bet you'd manage to get that done faster than improving Theora encoders to the point that they beat x264, and then we'd have no need to argue about whether something is patent encumbered or not anymore.