Coming down for no mana on turn one, it's quite possible to have the explosive start of Mana Crypt into a Signet or Talisman, land, and another Signet, leaving that player untapping five mana on turn two. In games going over twelve turns, the accumulated threat of damage from Mana Crypt provides a reasonable counterbalance for its explosive effect, but when you are snowballing to a turn-six to -eight win, it's a meaningless drawback.