Now after deferring the 3rd party scripts, we know for sure what the user experience is with regards to the main content of the site.
All that aside, though I'd say I just changed my mind to agree with you.
http://www.olark.com/spw/2011/10/lightningjs-safe-fast-and-a...
...this technique begins downloads immediately, but still avoids blocking window.onload (just ran a brief test myself to verify this behavior). Unfortunately, it requires the third-party provider to adopt LightningJS. Here's to hoping that more providers take notice :)
"Always" seems like an unnecessarily high bar. If any meaningful fraction of your users would be better served by getting readable content in 2 seconds and having to wait 10-90 seconds for third-party features, I would absolutely say this is definitely a worthwhile optimization. Even if it leaves some speed-reading twitterer waiting the same 10-90 additional seconds for those features to load and render.