Isn't poisoning generally considered an underhand and deceitful tactic? Something generally looked down upon by your hardcore do-gooder types?
Of course, once you've snuck past/illusioned/knocked out the guards you can just go in and beat the cultists to a bloody smear anyway, albeit with slightly greater risk than poisoning them first. Well, except that you're dealing with a contact poison so touching the cultists might get the adventurers weakened anyway.