While pets only have normal affinity effects, Killer Instinct functions as any other circle ability. It provides +15% damage done and -15% damage taken if the affinity is correct.

I have 5/5 merits in Killer Instinct because it seemed like the least disappointing option other than Beast Affinity. It's still disappointing due to the duration and recast, mind you, but not AS disappointing as Feral Howl.