"For injuries ought to be done all at one time, so that, being tasted less, they offend less; benefits ought to be given little by little, so that the flavour of them may last longer."

http://www.constitution.org/mac/prince08.htm

I've been in a situation where the company was laying people off incrementally over the span of 18 months. I hated going to work, didn't give a shit about the company, and I was not unique or alone in that opinion. If you have to lay people off, do it as quickly and with as much dignity as possible.

Morale wise it's probably better to do it in one large chunk instead of smaller servings where people are wondering if they are going to get cut in the next round.

A big reason is that it's much easier to do from a legal perspective. Letting go of low performing people in a layoff is way easier than going through all the hoops of giving them warnings, putting them on probation, etc.

Also, a slow trickle of firings is just horrible for everyone. You have no idea if you are next, etc. A big layoff followed by a company meeting of "hey, we did a layoff because of X. There will be no additional layoffs as long as we hit our current goals."