Poachers knife has several issues:
No sustain: Sustain junglers need mana (trailblazer provides this.) Most manaless junglers do not sustain well and need health (trailblazer provides this.) If one is going to gank or invade, one would typically rather be topped off on health and mana, a huge disadvantage to everything except rangers.
Can't smite your own jungle: While constantly counterjungling is nice idea, you simply cannot be in the enemy jungle all the time. You hop in, grab a camp here and there when it's safe and available, then go back to your own. Poacher's knife basically means you don't want to use smite in your own jungle since it will go on a longer cooldown. This is the same issue with the two items that turn smite into a combat summoner, it's never attractive to smite your own damn jungle.
If you take runic affinity, and the corresponding summoner cooldown in utility, smite has a 54s cooldown. The smite time difference is then 41s smite for trailblazer and 27s smite for poacher's knife, only a 14 second difference.
The gold bonus of poachers is pitiful, counterjungling a camp gives you the same ammount of gold as lane-taxing a single minion. How many camps can you expect to counterjungle in game, 15~ if that's the majority of what you're doing all game? 300G aught to pay for all the health and mana pots you need cause you didn't buy a trailblazer :)
P.S. One thing that might increase its viability is if you didn't have to kill the camp to get the smite cooldown and buff. IE if you're in the enemy jungle all the time you can just smite something every 30s to get the buffs whether you take the camp or not, and run around all stack with buff. See a blue buff, smite it and get a bunch of mana, etc.
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