Bobross already explained this in first fat garth deck discussion (glory to BP Shadowman) - there are heroes who don't need specific card.
Garth can get control board control with almost anything. Do I care if I cast sword thief or secutress? Not really, so I can pack both.
Fat decks are weaker when it comes to searching for specific card, eg when I need spelleater vs mage. But if I don't need specific card, they have their advantages, especially when you pack enough draw
Bookmarks