What is interesting to me is that, in the TNG episodes mentioned above where Beverly is in command and Troi is training for command, it seems the idea was that the way to enhance these characters and give them more to do was to let them do the same things the lead characters were doing, essentially letting them "play captain." I would think a better and more realistic way to let a character grow would be to focus on traits and abilities unique to that character and consistent with that characters purpose and position.
As an example, TOS (and, in no small measure, DeForest Kelley) gave us a vibrant and distinctive character in Dr. McCoy. The character was allowed to shine in many episodes, but never by simply mimicking the role of another lead character. I can't help but believe that if someone had ever come up with the idea of "Hey, let's figure out some way for Bones to be the captain this week!", Kelley would have been one of the first to say, "That doesn't make any sense."