I wouldn't think you would need the Excel add-in. I've been playing with for a few months. As I understand it, the Add-In as a way to use Excel as a client which can connect to the SAS server and do stuff.
So you if you had folks who loved using Excel (sigh) and you wanted to give them the "Power of SAS" (running SAS PROCs or Stored Procedure on Excel data and returning results to Excel), then the add-in is a solution.
But if you're a shop of SAS programmers, who plan to keep being SAS programmers just working on a server SAS instead of PC SAS, wouldn't think folks would be interested in the add-in.
That said, I would suggest installing EG. It's not a bad programming environment. Has some benefits (and downside) vs Enhanced editor. But of course you can do very well with just a good text editor and a terminal window...
HTH,
--Q.