If one isn't running on Windows, is there still a strong argument to be made for supporting OS threads over simply multiprocess (with some form of message passing)?

In what kind of situations would one really benefit from threads over processes? I'm genuinely curious, it's been a long time since I've run into blocking on 100% cpu in any kind of real world workload (doesn't mean they don't exist!).

I understand that we will need multicore support, I'm just not sure why we need multithread support?