He's not a hunter is he? I don't give a shit about his friend.
The only reason to get scarlet kris is if you want more resilience or plan on using instant poison.
It's because two people namely Lofi and Medanx are the only "geared" rogue and hunter so everyone just copies what they do. Not necessarily if it's BiS or suits your playstyle.
If you're using instant on a fast weapon then you're doing it wrong.
Instant is a PPM, which means it's more likely to proc on a slower swing timer. Crippling is percentage based, meaning the faster the weapon the more procs you'll see.
Glad I could help.
-movements