The fact that we can prevent sentinels from attacking by leaving off Assault is one of the aspects that gives them superior survivabilty. They don't go charging into enemy clusters, or draw aggro to themselves with the mod removed. Not equipping a weapon to prevent attacking isn't really an option, because of the way you get set mod bonuses from anything they have slotted(the mechanical behavior of it wouldn't be consistent anyway since none of the organic companions equip weapons). If anything I'd want an Assault equivalent for the other companion types too. With those though, they could make the Assault functionality inherent to the various special attack mods instead of it being a completely separate mod.