Sometimes you may be forced to use absolute urls. For example I had problem with FeedBurner's blog post preview page. Image links specified as relative urls were not resolved correctly. I can imagine some poor RSS readers may behave similar way.
I wanted site.url to be configurable, because I didn't want to hard-code final url there. I still want to be able to have working link when doing local web site development and testing. I have small bash script which runs jekyll -serve --auto with this parameter set for local development. My setup is more complicated, because I use several jekylls behind reverse proxy to simulate setup on http://binaryage.com which uses gh-pages from many repos together with layouts linked as git modules.