Not all. Here's one example of where live streaming beats progressive download:
You are a website which serves an extraordinary number of videos, most of which are only viewed by a few users (e.g. Facebook). A small percentage of these users want to view your video on an iPhone, so you re-encode it on the fly from the regular file on the website to display on iPhones. In addition, this lets you adapt the bandwidth on the fly to maximize quality without the signal dropping out.
You are a website which serves an extraordinary number of videos, most of which are only viewed by a few users (e.g. Facebook). A small percentage of these users want to view your video on an iPhone, so you re-encode it on the fly from the regular file on the website to display on iPhones. In addition, this lets you adapt the bandwidth on the fly to maximize quality without the signal dropping out.