Not being able to track changes is a deal breaker for me too. I work with lawyers quite a bit in the work I do, and as picky as they are with the way a document is worded, being able to track changes is a must.
There are a lot of other features missing in iWork and mobile office that make it unreasonable to use for anything work related. Plus, importing a word document into mobile office or iWork screws up the formatting of the document in my experience. Not worth the hassle IMO.
iWork and Mobile Office are fine for very basic document creation (never going beyond typing text), however in going beyond basic document creation or in needing to export the file outside of the iPad, they are lacking signficantly.
I was using Pages on my mac and after using the versions feature heavily during one late working session, I can see how this could be *very* useful if implemented on the iPad.