I don't post often - for usually someone else has already said what I'd want to say by the time I get to the comments - but I certainly lurk a lot. I find the friend/foe system useful for drawing my eye to certain posts - and I probably read the friended/foed posters more than the neutrals.
I try not to use this "power" lightly, since users I mark show up for other users too - it's not just whether I agree or disagree with a comment. I friend users who've said something particularly elegantly worded, insightful, or who somehow popped into my head and put to text thoughts that I hadn't even been aware I'd been having yet. As for foes... for those who diligently spread misinformation or whose posts seethe with misplaced malice, the little red dot reminds me not to overreact.
I do read the posts of foes, so as to bump them back up to neutral if it becomes apparent that I'd merely read the offending post when they were having a bad day or something. And when my lists gets maxed out, I go through and skim the recent posts of foes who are also friends-of-friends and return to neutral any who I have misjudged. I also return to neutral those userss who don't post often, as the mark would serve no real purpose. I don't think I've ever unfriended anyone, though.
I don't hold grudges... heck, the reason I use the system in the first place is because I can't remember that many names!
I proudly wear the Grammar Nazi armband, but I don't consider grammar at all when marking users. Wisdom doesn't *require* perfect communication skill, but it's surely enabled by it.
I don't accept moderator points (I don't feel I have the time and dedication to use them properly), and I wouldn't use the friend/foe system if I did... that smacks of ethics violations.