For those interested, the reason the old google analytics snippet wasn't using this instead of document.write:

There is an edgecase bug in IE6 that causes a dialog to blow up… under some security settings (unsure if they are default) when requesting form the non-'ssl' subdomain

So looks as if it's fine to use if you're not supporting IE6.

The new GA snippet uses different domains for the two modes so it can't be used.