I don't really understand the appeal of a friend/foe system. If I don't like a person, I typically try to avoid interacting with them unless/until I find a way to reevaluate them or myself so that I can find something positive, a way to like them. Pretty much the last thing I want is a computer to remind me that I don't like someone. In fact, I wish frequently that I could suppress my own internal feelings of dislike for people. When reading comments on sites like this I try not to read who posted them until I've evaluated the post on its own merit. The times I find myself pre-judging a post when I see who made it, I kick myself and struggle to evaluate its claims as objectively as I otherwise might.
I do understand that not everyone interacts with every site in the same way, and I personally do not approach every online community in the same way. So, I say, to each their own. The use of a friend/foe system seems to impose a bit of centralized structure, however, and I'm not sure how comfortable I am with the idea of enumerating to some website exactly whom I prefer as people, when I'm constantly trying to reevaluate and expand that in myself.
Slashdot is interesting in that it is one of the remaining bastions of (at least superficially) anonymous discussion. There are apparently several strata of users: some don't read anything by anonymous cowards, some don't read posts by their foes, and some only use the site anonymously. It makes me curious about how these different self-selected filter bubbles might give different perceptions of the discussion to different participants. Personally I have great difficulty engaging in public discussion without trying to read and understand as many diverse opinions as possible - I feel as though otherwise I'd be speaking out of ignorance.
EDIT: On occasion (at least speaking for myself) we don't like someone because they remind us of something about ourselves we try not to acknowledge or think about. Blocking their posts en masse can deprive us of an opportunity to explore this and grow.