The key is to ensure the content from an RSS or external feed only forms a part of your page, especially if it is a popular feed that is likely to crop up on a lot of other sites.
All things in moderation is the key. If you form a page entirely from a feed, with no modification and no other unique content, you are unlikely to rank that page. It shouldn’t affect other pages on your site. If you do what Sonic does, add your own stuff into the feed to make it a bit more unique, or mix the items up a bit, perhaps only show selected items, this is a good move. If you have lots of other unique content on your page, then a small feed of a few items can actually be beneficial in that it is showing freshness when the search engines come round.
Google wants websites to be offering the visitor the answers to their questions, but think of this in an “overall” context. Unfortunately, Google’s algos are far from perfect, but logically, if the page brings together useful information from different sources and the user can get everything he needs to satisfy his query, ultimately it is likely you will be rewarded for that.
One other thing you want to watch for is the fact that many feeds will come with a backlink built in, or a requirement to add one. This too can actually be a good or bad thing. If the link is to a quality site, on your topic, or a page relevant to your topic, that too can benefit you. If however it’s some 2-bit site, in a “bad neighbourhood” (spammer heaven for example) or right off your topic, then that’s a bad thing. So pick your feeds carefully.
Don’t think backlinks are bad – they can be quite the opposite if you choose who you link to carefully!
So my take: RSS/XML feeds can be very beneficial, if used sensibly to form a part of the whole.