Business Insider says that Marissa Mayer's work from home ban was supported by VPN data indicating that some external employees weren't logging in to Yahoo!s network. No login, no work?
While in isolation there's not a causal relationship, in this case the data probably doesn't lie.
Seems Max Levchin's comment that, in many cases "working from home" is not really working applies.
Since writing this post, I had the good fortune to by employed by a company with a very flexible work from home policy. All I can say from my personal experience, I get significantly more done at home than in a poorly designed, open plan office with innumerable distractions.