As enemy health scales to Hawke's level, Normal-ranked enemies can reach 1800+ on Normal difficulty by the time this ability becomes available. 20% of this is 360, which denotes the effective damage "value" of a deathblow, which would otherwise take several more basic hits or an activated talent to inflict normally. The value of this ability grows with the game's progression and difficulty setting (+30% health on Hard, +60% on Nightmare). Thus, even the "chance" for a deathblow is worth pursuing.
Elites have roughly twice as much health as their Normal-ranked counterparts of the same enemy type and class, thus 10% of their health hits roughly the same mark and confers the same "value" of a deathblow.