Layoffs result in a 30% increase in staff turnover. As markets tend to be more challenging, it can take some months before people actually leave. This can have a knock on effect, especially if senior and respected staff leave.
No they don't, because you never know if you're next. I have a bunch of friends at Google (you know, one of the richest companies on the planet and 94% 'survived' another day).
Many are scared. You don't know if there'll be another round and if you're in it. People aren't now magically more productive because they survived and their peers didn't.