Asynchronous IO is great, but in some applications it's not enough, and threads are better. This becomes especially useful in applications with GUIs. You can use Tk's "after" utility to simulate multitasking, but it ain't pretty. Using threads is much, much better, and far more comfortable.

Threads make it very easy and quick to share data between units of execution, unlike processes. When you have a manager-workers model that requires a lot of data to be passed around quickly, threads come in very useful.

Continuations won't come instead of threads. While they make "cooperative execution" possible, it's not what they really are for, as far as I understand. You still have to decide when to give up control, and with many paths of execution it's important that this is done quickly and efficiently.