I must say that to begin with, when I heard people ask for day/night cycles on each individual planet I was against it, much for the same reasons as Bioware have stated. But you do make a really compelling argument and you have sold the idea to me
I was a fan of the strong visual concept including the chosen time of day, the planets looked beautiful but yes some them always felt a little static.
After reading your post however I believe that day/night cycles will indeed improve the planets and I don't see any reason as to why the developers still can't have a strong artistic and creative input in the visual design. Each planet could for example have different night/day cycles and imagine the amazing and alien night skies that we could be spectators too. Furthermore the developers could , if the really want to, even choose the time of day (or night) that you land on a planet, when you visit it the first time, in order to convey the atmosphere in the strongest possible way.
However all this is no small development feat and I understand why Bioware chose not to include day/night cycles, as it requires a lot of texture work, perhaps even changes to the 3D models, new ambient score etc. etc.
That being said it would be amazing to see.