I know document.write is considered bad practice; and I'm hoping to compile a list of reasons to submit to a 3rd party vendor as to why they shouldn't use document.write in implementations of their analytics code.
Please include your reason for claiming document.write as a bad practice below.
There's actually nothing wrong with document.write, per se. The problem is that it's really easy to misuse it. Grossly, even.
In terms of vendors supplying analytics code (like Google Analytics) it's actually the easiest way for them to distribute such snippets
As long as you don't try to use it after the document has loaded, document.write is not inherently evil, in my humble opinion.
It overwrites content on the page which is the most obvious reason but I wouldn't call it "bad".
It just doesn't have much use unless you're creating an entire document using JavaScript in which case you may start with document.write.
Even so, you aren't really leveraging the DOM when you use document.write--you are just dumping a blob of text into the document so I'd say it's bad form.
I think the biggest problem is that any elements written via document.write are added to the end of the page's elements. That's rarely the desired effect with modern page layouts and AJAX. (you have to keep in mind that the elements in the DOM are temporal, and when the script runs may affect its behavior).
It's much better to set a placeholder element on the page, and then manipulate it's innerHTML.
Off the top of my head:
(1) document.write needs to be used in the page load or body load. So if you want to use the script in any other time to update your page content document.write is pretty much useless.
(2) Technically document.write will only update HTML pages not XHTML/Xml. IE seems to be pretty forgiving of this fact but other browsers will not be.
A few of the more serious problems:
document.write (henceforth DW) does not work in XHTML
DW does not directly modify the DOM, preventing further manipulation (trying to find evidence of this, but it's at best situational)
DW executed after the page has finished loading will overwrite the page, or write a new page, or not work
DW executes where encountered: it cannot inject at a given node point
DW is effectively writing serialised text which is not the way the DOM works conceptually, and is an easy way to create bugs (.innerHTML has the same problem)
Far better to use the safe and DOM friendly DOM manipulation methods
It breaks pages using XML rendering (like XHTML pages).
Best: some browser switch back to HTML rendering and everything works fine.
Probable: some browser disable the document.write() function in XML rendering mode.
Worst: some browser will fire an XML error whenever using the document.write() function.
(plus) it's the easiest way to embed inline content from an external (to your host/domain) script.
(minus) it serializes the rendering engine to pause until said external script is loaded, which could take much longer than an internal script.
(minus) it is usually used in such a way that the script is placed within the content, which is considered bad-form.
(plus) you can overwrite the entire content in a frame/iframe. I used to use this technique a lot for menu/navigation pieces before more modern Ajax techniques were widely available (1998-2002).
Here's my twopence worth, in general you shouldn't use document.write for heavy lifting, but there is one instance where it is definitely useful:
http://www.quirksmode.org/blog/archives/2005/06/three%5Fjavascrip%5F1.html
I discovered this recently trying to create an AJAX slider gallery. I created two nested divs, and applied width/height and overflow:hidden to the outer div with JS. This was so that in the event that the browser had JS disabled, the div would float to accommodate the images in the gallery - some nice graceful degradation.
Thing is, as with the article above, this JS hijacking of the css didn't kick in until the page had loaded, causing a momentary flash as the div was loaded. So I needed to write a CSS rule, or include a sheet, as the page loaded.
Obviously, this won't work in XHTML, but since XHTML appears to be something of a dead duck (and renders as tag soup in IE) it might be worth re-evaluating your choice of DOCTYPE...