A while back, there was this pie-in-the-sky idea which was really interesting but not too practical, called Semantic Web. It didn't really pan out because it turns out that annotating your sites with metadata is boring and tedious and nobody really liked to do it, and anyway, search and Bayesian statistics simulated the big ideas of Semantic Web well enough for most people.
The ideas behind it still stand, though, in the idea of microformats. These are just standardized ways of using existing HTML to structure particular kinds of data, so any program (browser plug-in, web crawler, &c) can scrape through my data and parse it as metadata, more precisely and with greater semantic content than raw text search, but without the tedium that comes with ontologies and RDF.
Now, these ideas are about the structured exchange of information between arbitrary nodes on the internet. If every recipe site used the hRecipe microformat, for example, I could write a recipe search engine which automatically parses the given recipes and supply them in various formats (recipe card, full-page instructions, &c) because I have a recipe schema and arbitrary recipes I've never seen before on sites my crawler just found conform to this. I could write a local client that does the same thing, or a web app which consolidates the recipes from other sites into my own personal recipe book. It turns the internet into much more of a net, and makes pulling together this information in new and interesting ways tenable. In its grandest incarnation, using the whole internet would be like using Wolfram Alpha.
The #! has precisely the opposite effect. If you offer #! urls and nothing else, then you are making your site harder to process except by human beings sitting at full-stack, JS-enabled, HTML5-ready web browsers; you are actively hindering any other kind of data exchange. Using #!-only is a valid choice, I'm not saying it's always the wrong one—web apps definitely benefit from #! much more than they do from awkward backwards compatibility. But using #! without graceful degradation of your pages turns the internet from interconnected-realms-of-information to what amounts to a distribution channel for your webapps. It actively hinders communication between anybody but the server and the client, and closes off lots of ideas about what the internet could be, and those ideas are not just "SEO is harder and people can't use curl anymore."
I don't want to condemn experimentation, either, and I'm as excited as anyone to see what JS can do when it's really unleashed. But framing this debate as an argument between crotchety graybeards and The Daring Future Of The Internet misses a lot of the subtleties involved.