Sorry I'm going a bit quotey on you here
That's not the interface I object to, its the interface where a user selects promotions for their units and the way those promotions are effectively misleading.
They're misleading only if you don't know how they work. Not knowing how they work is because of a documenation problem - it was not clearly explained in the manual (well... not even touched by the manual

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The rule is: Any time you ask a user to make a decision you provide them nothing but facts.
Agreed, unless those facts unnecessarly overwhelm the user with information. There are lots of little inconsistences all over the game that are not explicitly documented. Just look at the way it says that Protective leaders get double production speed of walls. In fact they get +100% on the base production, very different when the civ also has stone because it means it would only be one and a half times the speed increase (200% to 300%). I could complain about that in the documenation but I can see why they opted to just describe it as "double speed", even though I still personally disagree with that option. Another example is the way the March promotion is described in the documentation (can heal while moving). Many people mistakenly take that promotion thinking it means their medics will heal units as the medic moves!
I just think the issue with the promotion bonuses is minor or at least comparable to a huge number of other interface issues and I don't see a strong reason to pick this one out in particular as being ridiculously misleading beside all these other problems...
As I said I can understand how they got to where they are ... I could even see the spec sheet ... but I cannot condone them for leaving their users behind in the process. Now they could fix it in one of two ways. One is to actually label them for what they do and clearly explain the combat system ... but this will very likely confuse a lot of players (leading to an ever poorer interface)
Exactly, they simply should have given one or two worked examples in the manual and really there would be very little confusion for the players who wanted to think about the numbers when promoting.
or they can design the system to meet the user's needs while still retaining the level of playability they desire from the combat engine.
That may be easier said then done. I think the playability from the combat engine is a very high standard if you disregard any interface/documentation failings.
My instincts as a software engineer myself tell me that odds are they went the 1st path and then QA said "this is too confusing, simplify the labels" and never really considered the 2nd path because at that point the engine was written and would be too "hard" to refactor or more importantly rebalance and regression test. This is fine, concessions like this happen all the time in this industry, but that doesn't make it any less sub-optimal and as a consumer doesn't make me any more willing to accept it and not be critical of it.
Or they went along the first path and QA said it was too confusing so they decided to simplify the labels knowing that the number of people who would pick out the asymmetry would be few. Actually I think you are the first one I know who has actually criticised the labels as they are - I always thought they were a bit odd but I figured it was to help keep the information from being overwhelming.
I have no idea why you bring the phrase sub-optimal into this dicussion. Sub-optimal from the point of view of the most inspecting high-level players in the most significant minority of the player-base (software engineers who use civfanatics)? Oh please... I hardly think implementing the "optimal" path for these specific people is even worth a moment's thought for the developer, and I doubt they thought that was a sacrifice they needed to make.
Anyway, it's not my intention to start a quote war but I like the debate about the game mechanics. I guess I'm tired too which might be making my responses less coherent

. Really I think we have a similar stance on the problem with the interface here - the difference is you think it is more misleading than I would think to call it, at least to the average user (is it 15 year old gamers now?).