Disclaimer: I am not a DBA. I am a database developer.
A DBA just sent a report to our data stewards and is planning to remove the NT AUTHORITY\SYSTEM account from the sysadmin role on a bunch of servers. (The probably violate some audit report they received).
I see a MSKB article that says not to do this.
From what I can tell reading a variety of disparate information on the web, a bunch of special services/operations (Volume Copy, Full Text Indexing, MOM, Windows Update) use this account even when the SQL Server and Agent service etc are all running under dedicated accounts.