Hilmar,

>Are the Smart Tags, introduced in Word 2003, of any practical use?

Smart tags were introduced in Office XP. In Office 2003 this technology was enhanced and extended to all Office applications. Internet Explorer had support for smart tags too, but now it seems to be dropped because of security reasons.
Smart tags can be used to integrate Office with your applications or web services. It could be a good choice if your customers often need to use data from the application in Office documents. Two samples of smart tags are FedEx smart tags (http://www.fedex.com/us/ebusiness/smarttags/faq.html?link=4) and WorldLingo instant translation (http://www.worldlingo.com/microsoft/smart_tag.html).
If you will decide to develop smart tags then this article could be helpful: http://www.utmag.com/September2002/Page14.asp