> People who don't care much about Internet Explorer support often say to just use pushState, and let IE users endure full page refreshes ... but that's not really an acceptable way to build a responsive web app.
Why not? It's responsive for those with a modern browser (and may get the IE team to integrate the history API in IE10), and is less responsive for those with an older browser. That can be an acceptable tradeoff.
Sure -- it depends where you draw the line for "acceptable", and how much you care about the experience of your IE users. In any single-page web app of significant size, I'd argue that it's never acceptable to do a full page refresh. Would you really want to go back to a GMail that refreshed when you clicked around between labels?
Regarding (2): hash-based URLs also work perfectly well for older browsers.
you're right, "acceptable" is subjective. but it doesn't seem unreasonable that if you are using an outdated version of a program, it will not perform optimally. to that end, managing both pushState and hashchange adds overhead and complexity. personally, i don't think enhancing old browsers is worth said overhead.
that said, if it was part of a framework i was using anyway, overhead and complexity are much less of an issue, so the question becomes "why not".
Why not? It's responsive for those with a modern browser (and may get the IE team to integrate the history API in IE10), and is less responsive for those with an older browser. That can be an acceptable tradeoff.