[LDiCesare]

Not entirely. They are useful to bring in cursed daggers and lycanthropos amulets to enemy thugs. They are useful to kill regular commanders who would be able to lead troops out of a province where the only leader got killed. That's situational, but not a reason to make them even less useful.
They are also useful in forcing opponents to deploy bodyguards instead of fielding the troops where they are more useful.
I don't like making assassins totally useless in order to make commanders more useful.