Also versus Burn cards drawn, and turns played, are much the same thing. A hymn to tourach is a two turn timewalk when you force discard of two burn spells, same with an healing salve. This is what make casting pox viable vs burn. Consider situation where the opponent is on the draw, playing land and goblin guide with haste. Next turn you play land, dark ritual, pox. You go from 18 to 12 life, kills his guide, land, and force discard of two of his five cards in hands.
You yourself will have 3-4 cards left in hand depending on the result of the GG attack. A discarded nether spirit or bloodghast gives you the edge now.
Granted there are better times to pox but this was an illustration on the effectiveness of discard vs burn.