What? Dissipate does absolutely nothing against Zombies. If you hold up three mana, they don't play what you want to dissipate(Messenger or Gravecrawler, and honestly spending 3 mana to counter a gravecrawler is not a good deal), and instead lay down a Blood Artist or one of their other two drops and attack you. Or they play nothing and then throw a burn spell at you at the end of your turn.