Threads have been useful for me in my latest experiment. Its a C++ application that runs mame_libretro dll (copies) each in their own thread, while a BASIC intepreter runs in another thread, and the main game engine (Irrlicht) runs in the main thread. Irrlicht isn't multithreaded so I just put commands from the BASIC thread into an outgoing deque which I lock and unlock as I access it. Then there are mutexes for the video data.

I think that threads are definitely a bit tricky though since it is easy to mess up locking/unlocking or not lock things necessary and if you do then you have debugging headaches. So when not needed they should be avoided I think.