Progressive rendering is obvious if you have things below the fold. But it makes sense even if everything is supposed to be visible from the start. The total download+render time can actually become faster, not just because some things are not visible. This is because xmlhttprequest is truly asyncronous so you can parallelize the download and the rendering. This also prevents the "page seems to be frozen" warning most browsers pop up after being stuck in javascript for > 3sec.

I found this technique to make major improvements for a similar task. This only makes sence once rendering is a big bottleneck of course, you don't want to over-parallelize.