The halfling can threaten to end games fairly early with plenty of Ring-tempting options in black and white, and nearly all of them cost four or less
Tolarian Academy is arguably more powerful, producing one blue mana for each artifact you control, which are easier to generate and harder to kill
The artifact decks were often the best at this, because they brought out their mana so fast that they could shrug off losing two lands while they crippled their opponent's development, and decks with green mana creatures like Birds of Paradise often used the same plan
If we ban these cards and let the dredge engine survive, we are committing to banning the next Bloodghast, the Bloodghast after that, and the one after thateither that, or Wizards will simply have to stop printing powerful, graveyard-based cards out of fear of making Dredge too powerful