Well, if you are arguing some units should not be worth buying for thematic reasons, there's not much I can say to that. Because I can't think of any units that would fall into that category, militia are supposed to suck but they can also be thematically made much cheaper than other troops, light cavalry may suck but thematically/historically they have their uses even if it's hard to model in dominions.
In any case, I hardly meticulously compare every units stats while playing a game, it's 90% just a sort of intuition based on past games. The only place that really comes into play at all is when I have a number of very similar national troops, and really the only difference at all
is one or two points of defense.
And you can balance dominions for the next century but I doubt you will come up with anything resembling chess by tweaking units stats.
