Why not have two options: compressed and uncompressed?  If you have a simple
document, or a very complicated one with, say, a huge MPG in it, and you're not
transmitting it over a network, there's very little reason to compress.  It's
trivial to test and see if a file is actually a tarball.  
Of course the actual structures would be the same, just one would consist of
potentially several files in one directory and the other a single tarball. 
Many other office suites have multiple formats; MSOffice's "quick save" format
comes to mind.
--JRZ