I think the idea is to conceal the confusing CC feature from those who don't understand it, since everyone involved in the email chain must see the box and not just the clever originator who threw the CCs in there. That is, Adam sends out an email with all the bells and whistles, but Barry has no idea what all those random abbreviations mean and they frighten him. Adam and Barry both mostly just want to know who's going to receive this thing, so Gmail hides the CCs but shows the name. I think there's the unspoken belief that CC/BCC is a complete misfeature and Google wants it to die silently for the sake of clean UI.
It might not be the right approach, but I see why they did it.
The situation is actually a bit worse than that. As soon as Barry replies, Adam becomes the only person on the to line and everybody else is moved to cc despite all of Adam's hard work to distinguish recipient types up front. To vs CC is basically broken in threaded conversations. Moreover, unless you turn on some hidden options there is no visual indicator telling you who was on the to line vs the cc line in the Gmail inbox anyway. The distinction has become meaningless for the vast majority of cases.
Yeah, I think that makes sense (although I don't consider myself part of the group that has this unspoken belief about the misfeature of CC/BCC). But if they just at least keep the three separate fields visible to me as soon as I fiddle with them, they can please both the average and the 'advanced' (for lack of better word) user.
It might not be the right approach, but I see why they did it.