Great point! However, I've found that the implementations of user space, preemptive threads (UPT) have friendlier APIs than the implementations of kernel space, preemptive threads (KPT). Also, UPT APIs generally come with really nice synchronization primitives (like STM in Haskell) or designs that remove the need for them (like message passing on BEAM). Also, I really like not worrying about creating too many threads and having to use a thread pool because the overhead of UPT is generally so much lower.