The (largely untrained) economist in me says that mobility of labor is very important for growth and economic efficiency, as systems work best when the best people flock to the most important well funded ideas.
A dynamic pointed out quite explicitly by Adam Smith in Wealth of Nations.
https://en.wikipedia.org/wiki/The_Logic_of_Collective_Action
> it's net neutral for most companies
My take is that if there were only two companies this would be true. Considering that no company employs more than a tiny fraction of the skilled labour market the vast majority of labour innovation occurs elsewhere.
A company would have to be doing an immense amount of innovation in order to not have a net benefit from free movement of labour.
We could talk about whether the Googles, NSAs, or Amazons of the world hit this threshold, but I'd argue that nobody else does.
Of course this would probably reduce the number of HR roles but hey you cant make an omelet without breaking some eggs :-)
Worth noting: much as I despise Facebook, Zuckerberg did the Right Thing here and didn't play along (though it's not clear whether or not FB reported the collusion).