Hm. I've worked on Paranoia games that had 3 GMs and 14 players....
The major reason to have multiple GMs is to allow "parallel processing" - having the ability for GMs perform multiple actions at once. This comes at a cost of having to spend time communicating between GMs. So, there's a balance. If having multiple people working saves you mre time than the communication between GMs, assistants can be an asset.
In most games of less than... oh, 6 PCs or so, you probably don't generally win using an assistant, unless the group splits up. Otherwise, your assistant winds up best relegated to scutwork of setting up battlemats and running the occasional NPC...