Nah, the reason open-plan offices are popular in Silicon Valley is that SV has embraced a modern truism that the rest of the world recoils from: most of success is luck.

Open-plan offices optimize for luck (as in, chance encounters that spark new ideas) at the expense of pessimizing known, guaranteed, solid productivity. Most people won't be as effective in heads-down, concentration-required, focused, knowledge-based work in an open-plan office. But this is irrelevant: in today's economy, if you're doing something that anyone else knows how to do or that anyone else thinks is a good idea, you're a rounding error. The only work that generates any financial return is when you do something that everybody else considers too crazy to try and it works.

Open-plan offices in startups are usually because the founders hope that enough smart people in close proximity, dedicated to working on a problem, will manage to fill in the holes in the business plan that the founder didn't get to before they raised money. Open-plan offices in big companies are usually because management hopes that employees will generate the next big idea that keeps them from becoming obsolete.

There's a lot of cargo-culting going on too, and particularly in other parts of the country, people adopt the open-plan offices without the incentive structures, openness to new ideas, and ability to bring a product to market that allows companies to capitalize on it. But that's why Silicon Valley companies do it, at least.