Perhaps I wasn't specific enough with "MMOs." I'm talking about the kinds of games where players can create their own maps, with their own scripted NPCs, and push them onto the server, where other players will then pull them down and execute them in real-time. Second Life, MOOs, etc.
This is basically an equivalent model to web-browsing. You don't load compiled Javascript into your web browser's process address space and jump into it; you put an interpreter between you and it, both for the sake of the code (JS has its own debugger, which treats the browser as a black box that scripters rarely have to think about, and never see backtraces from) and for the sake of the web browser (which doesn't have to deal with any consequences of bad scripting affecting the validity of the browser's own data structures.)
This is basically an equivalent model to web-browsing. You don't load compiled Javascript into your web browser's process address space and jump into it; you put an interpreter between you and it, both for the sake of the code (JS has its own debugger, which treats the browser as a black box that scripters rarely have to think about, and never see backtraces from) and for the sake of the web browser (which doesn't have to deal with any consequences of bad scripting affecting the validity of the browser's own data structures.)