Foulmire Knight is kind of crappy, yeah, but its crappy in a way that a lot of decks are happy with it (lower-level depending, of course). It's a one-drop for aggro, if you like that sort of thing, and the deathtouch makes it an effective roadblock in slower decks. You can run it out on turns one, three, and four, so there's some great mana value flexibility as well. You're absolutely right in that it's almost never going to be something you're going to be pshched about running, but it has a lot of potential uses (holds equipment well, it's sac fodder that replaces itself if necessary, it plays really well with any Regrowth-type effects, etc.), which makes it a great little glue card. You'll almost always be happy seeing this come around 12th pick if you're in Black and it stands a lot better of a chance of winding up in a main deck than other "include it to hit your one-stop count" critters.
Zombies are also a lot more likely than most tribes to get incidental synergy with lords that are fine on their own, so that can be a very minor plus if you're into that sort of thing.
