Bad card decks....
Ya gotta love the crap Wotc comes out with to balance their cheese.
Like ignoble soldier from MM, 3/1 white creature that deals no damage if it is blocked.
Or moulting harpy, with an upkeep that totally negates it's speed.
But those are recent, there's also root spider, from homelands, a particularly crappy card, with very little usefulness.
Now I know that most people consider all homelands cards to be crap, except for baron sengir, autumn willow, serrated arrows, etc. but the only problem with the cards were that they had a limited range of uses, so that would probably be the definition of a bad card. Either that or one with a prohibitive casting cost.
IMHO there is no such thing as a totally useless card, but there are a few with such totally limited uses they come close.
Also I must agree with the others, raging goblin, guerilla tactics, and particularly fireblast are too good to go in a bad deck.
And as for the purpose of a bad deck, it is twofold. First it is incapable of making 'hiccup' games, games that last about as long as a hiccup. And second it can force you to actually think about your plays.
Oh yes there is a third reason, they can be just plain fun.
"Aha, you're playing blue, my sea troll is now much more powerful."