As per my understanding, the decision lies with business requirement and Criticality of dependent Applications which might be using the SQL SERVER. I personally feel that many organisation are reluctant to upgrade may be because of the Major dependencies of their Existing \ OLd Application. Migrating or Upgrading to a higher version let say, SQL SERVER 2005 Or SQL SERVER 2008 is not very complicated but the Complex \Rigrous Testing has to be done at Application Level to ensure that the latest version will work fine for their business Critical Processes.
No Doubt Latest Vesions have more Enchancements both in Terms of Development and Administration and ofcourse new Version will make the database management a lot easier .
So, the decision has to business Oriented which should help to make the Organisation Business more Fruitfull.
Sachin