Isn't it just an Opt-In, meaning it's redundant (not harmful) to use it on pages containing hashbangs?! In my understanding the crawler looks automatically for the _escaped_fragment_-page, if it detects a hashbang in the URI. Only if there's no hashbang in the URI you could advise the crawler to append the _escaped_fragment_-parameter by using the meta-tag. That way you can serve different content if the parameter is set. That's the way some blogspot-themes are working (e. g. http://illegalcartoon.blogspot.de/).
Did I get your question right?
Update:
The AJAX crawling scheme using _escaped_fragment_
has been deprecated by Google. Here's a quote from the regarding blogpost:
Today, as long as you're not blocking Googlebot from crawling your JavaScript or CSS files, we are generally able to render and understand your web pages like modern browsers.
Read the complete article here: https://webmasters.googleblog.com/2015/10/deprecating-our-ajax-crawling-scheme.html