The problem is that people don't socialize at all out of work still have those pesky communication needs. And they use meetings to fulfill them ... slowing everyone down. And what used to be one sentence affair is suddenly 30 minutes long hard to stop chat, because lonely people need to talk. Or one person job is suddenly two person job for no reason other then one of them lack socialization.
And since they dont mind spending there 12 hours a day, they organize things in a way that makes it impossible for other people to come home sooner. Meaning that meetings are late, important information is passed late etc.
And oftentimes means that tired people make more bugs and do less rational decisions. So they insist on work that does not need to be done right away to be done right away for no reason other then fear.
If someone works 12 hour days, and then implements the, and it hurts me to even type it out, "lifestyle hacks" to further squeeze the sponge of productive hours, the sacrifice they make to their own personal lives is a big toll. 12 hour days, plus commute, plus 8 hours of sleep leaves little time for socializing, hobbies and let alone exercise.
But the bigger tragedy is, on a team where there might be competition (eg., a bonus), the guy pissing his life into his work is going to get the accolades and bonuses, or at least it looks that way, which takes a morale toll on the other members of the group. It leads to a race to the bottom.
This might be okay when you're junior, first starting out, or a partner, or someone with significant equity (read: in a few years they won't need to work), but for everyone else, there is a very high risk of a toxic work situation.
To the person above who said that we're highly paid and so forth and expected to perform - well we're not paid because we're expected to perform highly, we're paid because laws supply and demand at present benefit us. If a situation should arise where the balance of the scales reverts, I assure you you'd still be completing your beloved 12 hour days with a much smaller salary.