Even for large sites this (combined with other rails/ruby improvements) is a huge gain. Because each rails app server can only handle a single request at a tim

Even for large sites this (combined with other rails/ruby improvements) is a huge gain. Because each rails app server can only handle a single request at a time, you have to spawn more and more instances of rails. Each of these instances consume 50-200MB of ram. Being able to have a single app server per server means I can stop spending so much on ram (or use it for other purposes). So at a large scale, this will be a large impact because it will reduce the number of mongrels/thins/... you have to run.