This was Symantec's reasoning:
Interoperability with Norton AntiVirus for Windows NT and
Norton AntiVirus Corporate Edition
Auto-Protect/File System Realtime Protection
If Norton AntiVirus for Windows NT Auto-Protect (NAVNT) or
Norton AntiVirus Corporate Edition (NAVCE) is running on
the same computer as Symantec AntiVirus/Filtering for
Domino, make sure that NAVNT or NAVCE loads before
Symantec AntiVirus/Filtering for Lotus Domino is loaded.
Symantec AntiVirus/Filtering for Domino is aware of NAVNT
and NAVCE operation and prevents virus detection
conflicts. In some cases, you may need to configure
NAVNT's Auto-Protect or NAVCE's File System Realtime
Protection to prevent scanning of the TEMP directory that
is used by Symantec AntiVirus/Filtering for Domino.