Q1. Why are there a number of plug-in class implementations,why not have just one class or code to rule them all?

ANS: Not that it is not possible,but some of the RSS feeds have have different methods of publication,some of the items may be different,some of them may use enclosure etc.,some XML CDATA etc. so to get the best results it was better to target feed by feed on the assumption that there may be some difference between feed.Some sites may have RSS , some ATOM etc. also.All plug-ins already done can be used as examples for furthering your own development.

Q2. Why does this application even exists?

ANS: