Finally, an answer that (sort of) makes sense.
A while back I was trying to figure out why the Professional Editions of Office 2003 are required for end users of VSTO solutions. After yet another post in the newsgroups on this question, I finally got this reply from Microsoft:
VSTO requires XML support within Microsoft Office and only the Professional
Editions of Office 2003 and the standalone editions of Microsoft Word 2003
and Microsoft Excel 2003 provide the XML support required for VSTO.
At least end users can buy the stand-alone versions of MS Word and Excel if need be, but I sincerely hope that the XML support mentioned (still not sure how the XML support relates to VSTO, but I'll take their word for it for now) makes it into the Standard editions of future releases, as that is what most companies purchase for most of their employees. I like working with Office documents in .NET, but if VSTO is going to be a viable technology, the end user requirements are going to have to be more affordable.