What is an advantage or disadvantage for not upgrading database server?
If you don't:


it will be unmaintainable at some point because of OS, version, patch, whatever
hardware failure may force an upgrade at the wrong time


What you gain:


older code can be simplified with new constructs (eg ROW_NUMBER)
far better error handling (TRY/CATCH)
engine improvements: most queries will run quicker on a later version


MS don't support it anymore either