Google has been crawling AJAX'd websites bypassing the _escaped_fragment_ mechanism for at least a year now.
I've never seen an example where Google crawled a page and included content that was loaded via ajax or an external javascript file. I have seen Google index content that is loaded from a script tag included in the html page.
However, from working with customers of BromBone, I know this. People have javascript powered webpages that were not getting indexed by Google. Google would crawl the page, and see no content. The pages were not in Google. After creating prerendered snapshots and serving them when the _escaped_fragment_ parameter was present, the pages showed up in Google.