I was thinking of the harder matchups, such as Burn, and wondering why we don't run Phyrexian Unlife. One of the hardest parts is managing your life total. At 3 mana, it's still relevant and since a lot of decks don't run a lot of enchantment removal, it essentially reads as gain 10 life. Furthermore the poison counters don't start until after you've gone to 0 life, so at worst you gain a couple turns.