I believe it's the fact that the user is not logged on that is the problem for Microsoft (and our Prod Support Team). I have also heard of people running into memory problems when office apps/drivers are used on servers for automated tasks such as this.
To be honest we have been running with this driver on another server for over a year without incident. It is only because we are moving to a new box that the issue came up. FYI it is the paragraph under "Additional Info" at the end of this page that was the sticking point:
http://www.microsoft.com/en-us/download/details.aspx?id=23734
The OpenXML solution is working well. We had a problem initially because we are copying a stub Excel file as a template before writing to it and we sometimes got file already open errors when we tried to write. We think the anti-virus system was grabbing hold of the file as soon as it was copied so we had to add some code to check whether the file was already open before writing and if so wait a while.