This unlocks the possibility for serendipitous connections. It is awesome when you can meet a founder, a leading engineer, an investor, a designer and a PR person in a coffee line, forming a new company before finishing your morning coffee.
Talents concentrated in one place gives significant boost to their productivity. This is called a "proximity bonus" in video games; building dependent factories in the nearby slots gives them the productivity boost. This is what US is missing by being nimbyistic.
I think we should return to the 1950s ideas of a futuristic cities and invest in building giant arcologies, where entire Earth population can move if they want to, and where the least paid McFastFood worker would afford to buy a minimum apartment.