First off, I'd suggest you not worry so much about PageRank when thinking about guest blogging. Google even said "PageRank is no longer—if it ever was—the be-all and end-all of ranking." ([googlewebmastercentral.blogspot.com ])
And like some of the other responses have pointed out, links in a blogroll can create hundreds of links from one domain that all use the same anchor text--if you have too many of these it might catch the eye of Penguin and land your site in hot water.
And you have to ask yourself, when was the last time you clicked on a link in a blogroll? It might provide an inbound links but are they driving any actual visitors to your site?