The main advantage for Quake 3 was that the server, client and UI code could be compiled into platform-independent bytecode modules. This was mostly a boon to mod creators, who had to build only one version of their mod that would work on all Windows, Mac and Linux systems alike.
I thought I heard this was also a boon for the scenario creators and level builders working alongside the graphics engineeers.
They get something a little less hairy and volatile to work with, and execution speed isn’t that critical because you’re only making hundreds or thousands of decisions per second.
