SEO - True. There are many different strategies to deliver solid SEO results using technologies like Spar. You can use headless browsers to pre-render and deliver on-demand rendered content, if you know your URL space completely, you can pre-render all documents. Spar isn't an attempt to solve SEO for you, you may not even care if you are building a phone-gap app, for instance. So does that invalidate the technology?
Accessibility: I'm 100% behind accessibility, and if the compatible tricks don't work... do you still make your site accessible for IE3? No... screen- readers MUST catch up. We can all do what we can to help, but shouldn't engineer for the past, which is an orthogonal concern from accessibile technology choices.
Page-performance - I'll challenge the several orders of magnitude claim, we've seen incredible performance w/ the approach, and best-case you're only talking about first-page speed, even ceding that it becomes a much different story later on... and if you're building an 'app' thats a big difference.