I added RSS widget for Multiply and found out that WordPress supports only link-tags. I had some debate with Multiply's admin and they reffered to RSS specifications at http://blogs.law.harvard.edu/tech/rss where it is said:
"In some systems, link is a permalink to a weblog item. However, in other systems, each item is a synopsis of a longer article, link points to the article, and guid is the permalink to the weblog entry. In all cases, it's recommended that you provide the guid, and if possible make it a permalink. This enables aggregators to not repeat items, even if there have been editing changes." (Technology at Harvard Law, Internet technology hosted by Berkman Center, RSS 2.0 Specification.)
I am not an expert but I just want to inform you what could be the reason why some RSS feeds does not work on RSS widgets.