Higher densities will enable more efficient public transit, greatly helping our environment, and helping people get their commutes back under control.
Also helped will be future generations that will get to live in denser communities, with more neighbors and more kids that their children and be friends with.
And of course it will help in the walkable neighborhoods to give more people access to them.
And I think you under estimate the ability of more duplexes to moderate the most exploitive aspects of the housing markets. In demand, high expense areas have prices that rise spectacularly, turning every single family home owner into a speculator.
Ironically, developers of duplexes and fourplexes are not speculators, certainly not to any degree that the homeowning residents are. Developers perform a service, enable more people to live in a place. The residents just get to benefit from others' hard work at making an area more desirable.