When I've seen layoffs good people often do depart because being in a company, never knowing if you're next to clean desk, is very stressful. Some simply don't wait for the severence, but proactively find somewhere else to go. It's all part of the risk that the company may lose a keystone employee here or there.
I'm seeing this happen at my company right now. In fact, I turned in my notice today after getting an offer from a startup. Back in November, we laid off 5% of the "dead weight" or those who had scored low on their annual reviews. The thing is, we really didn't have much dead weight, and the distribution to determine the five percent was very forced. Basically, management said thou shalt cut 5% of your engineering staff for each and every department. Since then, we've lost three device engineers, half the layout staff, and one of our principal engineers was spotted running around the parking garage making suspicious phone calls. I'm getting out before I piss someone off and land on the "refresh" list.
