Well, in my view security is not a binary, nor is there a silver bullet which renders all software secure. Security is a strategy in which one implements as many known protections as reasonably possible in order to reduce vulnerability. In the Google Docs case, I think there is absolutely no good reason not to authenticate HTTP request for components of a document that also requires authentication.

Sure, crypto-strong nonce URLs are better than unobfuscated URLs, but that's just one protection.

For example, let's say that Google accidentally leaks a directory listing of all files on a particular google docs server (or let's say an access log, to be even more realistic). If Google had properly authenticated access to these images, then the leaked access log would be worthless and unexploitable. However, without authenticating these requests, Google has suddenly caused a mass security breach, as hundreds of people rush to wget all the URLs before the images are taken down.

If Google is seriously expecting people to use their product as a replacement for a business intranet, then they need to take seriously all concerns.