Now before I continue, I must remind everyone: X-Plane is already multi-threaded and will take advantage of multi-core hardware. How much we use those cores depends on the type of scenery loaded.
The problem is that multi-threading (as a way to use multi-core hardware) is a solution technique, not a problem statement. What is threading going to be used for? If I simply program the other 7 cores of your computer to calculate PI to 223,924 digits have I met the feature request? This probably isn't what anyone wants.
Implicit in the request for multi-core is (I speculate) a request for better frame-rate. (I did see one user who wanted multi-core to be used for a more accurate flight model. This strikes me as a poor trade-off for hardware based on my understanding of the flightmodel - we would use a lot of hardware for only a marginal accuracy improvement - but I commend the user for stating the problem and not just a possible solution.) But is multi-threading the best way to get framerate?
If I had two patches to X-Plane, one that doubled fps by using two cores and one that doubled fps by using more efficient code, which would be better? To me the obvious answer is: the code that is more efficient. It will run on any hardware (not just multi-core) and if you have multi-core hardware, we still have that second core free for some other functionality.
So to me the feature request should be something like: "higher framerate - and yes I have multi-core hardware". Or perhaps "more visual detail at the same framerate - and yes I have multi-core hardware".
All feature requests need to be in terms of problem statements, not possible solutions. This lets us find the set of problems that can be solved together in a coherent manner, and it lets us pick a solution that meets our engineering goals.