This is my point. You would have to use normal units to kill your way through every other unit in the stack before you could reach the mage. And by then, you could attack him with a normal unit anyway. What do you need an asassin for? If everyone can get guardsman easily, the asassin just becomes a normal unit with poor city attacking capabilities. Aggressive gives a free promotion that most people are getting anyways. It gives a headstart. It also makes all your units come out of the gate slightly more advanced than the enemy's. Combat I is significant because it gives extra strength in all areas, making a unit more competent on the whole. It's also a prerequisite to many things, meaning aggressive civs can specialise their units right out of the gate, rather than having to do the basic training first. Guardsman is a specific thing for a specific situation. And it's also quite hard to get. Hence, the bannor excel at something in particular, protecting the weak. If everyone could get guardsman easily, it would cheapen their trait.