Is it good that employees be sacked without assigning a reason?

I think that it depends on the circumstances. If the person is losing their job because of something that they have done or not done, then no, they don't need warning. If they are losing their jobs through no fault of their own, then I feel that they should get as much warning as possible so they can try to find another job.
Here, one factory is closing in July, but everyone was notified before Christmas so they had plenty of time to try to find something. I think that this compsny did the right thing.