I'm not sure if I use the friend/foe system the same way as most, but I find my system interesting and useful, and thought some people (possibly those on my friend/foe lists) might be interested, too.
I'm not interested in keeping a list of people I don't like, nor am I interested in keeping it visible for the world to see. Opinions are generally personal, and may not be valid for others. If there's someone I really don't like, I'll remember their name and ignore what they write. There aren't a lot like that, and some of them might be on my foe list. I can only think of one, but as I said before, I'm not interested in publishing my (at least partly) emotional biases.
I'm also not interested in having a list of people who say the same things I believe, or modding them up and (further) distorting my view of the world.
So what can one do with these friend/foes lists that doesn't fall into those categories?
Well, I use friends almost normally. These are people I generally agree with, and who have comments that I find interesting. I don't add mod points - if I'm going to see their comments, it will be because someone else thought their comment was interesting, too. The important part is that they're interesting.
I use foes for a different purpose. These are people I generally find challenging my beliefs, though not necessarily offending me. I mark them because they've made me think about things more than once. They, too, don't changes to their mod points, for the same reason that I don't add them to my friends.
So if you see I'm on your foe list - the odds are good that you're there because I wanted to make sure to give your latest comment the attention I've decided you deserve from previous experience.
And if you're on my friends list, I'm sure I'll be seeing another comment from you that will bring me pleasure of one kind or another soon.