Even some fairly new software has its files blocked from Office 2003 as well, such as Word 2004. The reasoning? Security of course. Microsoft cites security concerns over older formats of documents creating a hazard, and for those still wanting to access old files they suggest a registry-hack workaround. A registry modification just to open old files? That seems a bit ridiculous – with how big they are on check boxes you'd think it would be trivial for them to include such a feature.
For people and perhaps businesses who have large amounts of documents in older formats, they might end up turning to alternative office suites. This decision from Microsoft seems poorly guided at best.