If companies are using mass layoffs to get rid of the "chaff", so to speak, then they have a management problem. Maybe all these companies performing mass layoffs do really have a management problem and hired way more than they should have. But if that's the case maybe the first step is to rectify the management problem rather than to layoff 10-20% of the staff. There really isn't a way that can be done smartly so that you retain the best people and don't destroy morale, effectively harming the shareholders in the long term.
Depends on who's talking/the situation. When you hear people talking about not closing a profitable factory instead of laying people off to delocalize, that's pretty much what they mean.
this is really hard to quantify, especially when it's increasingly becoming standard operating procedure across the industry.