I've read before that using multiple servers like this can increase load times for visitors as servers will limit the number of parallel downloads. This will only be noticeable on media-rich sites like image galleries or a YouTube like site. It also generally only benefits larger sites.
There can be potential downsides though from using certain servers. Some of the image servers people have used are blocked by my work place and thus half the site won't work for me (don't remember which servers these were). I also occasionally will come across a site where the CSS isn't working properly and I find it's because the CSS is hosted from a different server than the site, and that server is being blocked, sometimes by me using extensions.
So if you have a large site with a lot of media-rich sites, then yes using multiple servers can increase the speed for downloading pages, but depending on the servers you use, some may not be able to view the site properly, and they won't know why.