Why can't terminate a process if its threads are in unkown states?

Question

From my experience, when main thread is ready to exit, it should wait until other threads normally exit.

But from this link http://msdn.microsoft.com/en-us/library/ms686722(v=VS.85).aspx, it looks when process is terminated, all related resources are freed, so if certain worker thread is doing heavy work, waiting may be a litter longer. Can I ignore the waiting?

Also in the above link, I find

Do not terminate a process unless its threads are in known states. If a thread is waiting on a kernel object, it will not be terminated until the wait has completed. This can cause the application to hang.

This is too short to understand why killing a thread in unknown states when process exits is wrong.

can someone give me more detail about the problem?

Thanks