Duff-Man,
In an ideal world, (i would think) they would code certain parts of the game in seperate threads, anything that was needed to be tied together in one set, things that could be split into others. I know it would be nice. but like you say, its seldom it works that way.
We have had multi core cpus around for some time now, and i know its a lot more effort programming multi-threaded apps. Cant say i speak from experiance as i am not a programmer.
Car comparison is a bit ott, i would never expect a multi threaded app to run twice as fast as a single threaded app. As a thread could be held up waiting on another thread to complete what its doing.
It has the "potential" to be faster, or allow additional content though. Ai could be written as a seperate thread, on a multi processor based machine that would see a good boost. But then it comes back to the argument of what if something is held up waiting on the "ai thread" for example to finish its routine.
Meh i dunno lol