This is a great article, Rebecca - really interesting perspectives and beautifully written (not at all stodgy, IMO). (Postgrad student of Language Policy and Planning here, btw, so this actually useful for my own work as well - thanks!)
I have a couple of thoughts, which might not be of use, but are useful for me to try to express.
Re: 'usefulness' - this is one that bugs me every time I come across it. I learned French to fluency when I was younger. I have tried through my life to try keep it up by speaking it at every opportunity, continuing to read it, and so on. But my 'use' of the language is minimal. Don't get me wrong - I love that I know the language, and being able to read Zola in the original and so forth has enriched my life. But I don't use it much. But I use Welsh every day - to me, living in Wales, it is useful, and for communication. Yes, I can communicate with the people around me in English, but it is a different experience doing it in Welsh. For me, the usefulness of that medium of communication is that it strengthens the bonds of belonging and connection and group identity. To a monoglot English speaker, there is no difference to walking into the Mochyn Du in Cardiff and asking for two pints rather than 'dwy beint o gwrw, os gwelwch yn dda' - but to a Welsh-speaker the communicative experience is different. The refusal of so many English-speakers to consider communication through the medium of Welsh to be valid and meaningful is what upsets so many lovers of the language.
[More ramblings from me on similar aspects of Welsh-speakers' identity will be published in the August edition of Planet Magazine ... plug, plug]
Also, coincidentally, I came across this blog post today via Twitter - thought it was an interesting additional perspective: