Presence of Mind on Non-mana champs

Why are Rengar and other non-mana champs unable to take Presence of Mind? I get that they can't benefit from the mana refund, but the 10% reduction on the ultimate every takedown HEAVILY outweighs any other benefit that you can get from precision for that champion. If Rengar could take it, he might actually be decent in early game if he doesn't get a kill off his first ultimate, but as it stands right now, he'll basically lose any game when the enemy has a Warwick or Skarner who takes the rune and simply is more useful.