The reason that there are some situations where this environment variable is set by default is that some products which depend on eDirectory require it. The 'xad' reference is for Domain Services for Windows (DSfW) which is part of Open Enterprise Server 2. Current versions of the ndsd script also reference AFP as the AppleTalk File Protocol has a similar requirement. For backward compatibility Novell has not yet made this a default for all situations and as a result leaves it up to the administrator to set this per server.
HOWEVER, this should NOT be set in /etc/init.d/ndsd directly as that file is owned by a package. The reason this is a problem is that subsequent eDirectory patches which include that package (most of them) will undo the setting leaving you in the situation where you were before making the change. To properly set things like this you should use the pre_ndsd_start script which is "sourced" before the ndsd script starts eDirectory. The TID has been updated, as have various other TIDs and documentation with the same incorrect recommendation. This same recommendation (do not modify package-owned files) applies to all files owned by all packages in any package-management system. There are usually ways to implement customizations in a way that will not be overwritten by later patches and those should be used in all cases.