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>.tar.gz is not that uncommon, I would say.

On Unix, as tar has been built-in to Unix for decades.

And therefore OSX.

And I would say that if you are attempting to open an archive on Google Docs Viewer, it's probably an archive of documents, and I would say that almost all archives of documents are in .zip or .rar.

It is uncommon to the regular user, though, who is probably the one targeted by this feature.