I don't agree with system team, a DBA role shall not be narrowed to just instance of sql server,I think both the teams work together to get best out of the box. If someone thinks that DBA should have limited access as low as DBA can't make the system down, it mean's you shoudn't expact from that DBA can do much. I am assumesing that the DBA is not just a development DBA or Application DBA.