Where does it say you should use it instead of the exists plugin? I don't see it that way. all_entries reduces the number of entries the rss plugin generates per run, but that doesn't mean to me the same file may show up multiple times for whatever reason. For those cases the task could see all occurrences of the same file as "new" entries to the task, and that is when the exists filter plugin would kick in and block the same file from being downloaded again.
Also, I don't know how the limit_new plugin works, but say there are 55 new entries, with your current config, does it download 50 items, and then run the task again, in which case you get the 5 last ones? If you in this case would use "all_entries: no" it may be the rss does not produce new entries for the task for the 5 last items, since the rss plugin saw them before. Using "all_entries: no" may be a problem for you in this case.