I've been through 18 layoffs since 1996, about 12 of them while in management. I can only speak to the Bay Area - practices may be different outside. You are correct, that lots of times positions/jobs are made "redundant" as part of the layoffs - but speaking as someone who both observed, and participated in the process - those "redundant" positions were quickly backfilled after the layoffs if there was any need.

The one exception might have been when the entire browser division was dropped back in Netscape - everyone was chopped there - but I can't say with certainty whether low-performing Server Division people were impacted (though IT and HR positions were chopped). So - fair, when a division or operating group is cut wholesale, low-performers in other divisions might not be dropped - but knowing the mindset of management - they really like to take advantage of a layoff as a "get out of jail free" card to let someone go. Much less stress, and way, way less paperwork.