I was wondering why ad\'s still use the document.write approach to inserting the add into the page
I stand corrected, doc.write created scripts are blocking - worse than I though heh :) - but as an adblock avoider it's really weak, so I can only conclude it's an SOP mechanism for dynamically adding params to a script request overused.
Use the DOM insertion technique when avoiding script blocks kids.
I don't know for certain, but they might use it so all the content on the website is loaded and shown to the user first, then the ads are loaded and shown.
A traditional script tag will block the page while it is loading and executing. A script loaded with document.write will work asynchronously. That's why you see this on ads or analytics, as such scripts don't influence the page content directly.