The efficiency gain with open offices is that, for high-growth companies -- or ones that are uncertain about their long-term size, it's pretty easy to shuffle furniture around to squeeze in more people. When walls and doors get involved, you're committing to "each person gets X sq. ft", for a company that may grow from x to 2x employees.
Often teams that go from X to 2X end up getting less stuff done. There is an argument that startups should grow as fast as possible to pre-pay that debt, but getting less stuff done while burning 2+x the money is only seen as a good idea in SV.
And that possibility is worth more than letting people get work done? I mean, if people don't get enough work done, you're never going to get to the point where you need to shuffle things around.
They'll get the work done, at the expense of their sanity.
Also, market inefficiency works here too, I think - it can take years before a company tanks. So productivity drop can go unnoticed forever (even when a company fails, there will be enough other factors to blame). Not to mention that marketing can successfully paper over all kinds of crap; mediocre is the standard in our industry.
That said, they're a disaster for productivity.