Not disagreeing with anything in this post, but I think there's a difference between "need" and "could use." By pretty much all accounts, Mountcastle's bat would play at the major league level today, and probably would have months ago. On the other hand, I think pretty much anyone could use developmental space to focus on particular areas of his game without worrying about hurting the team, and that's even true for Mountcastle as a hitter.
I only say this because I can see a rationale put forth by the O's that they don't want to promote him for a developmental purpose. It would likely be cover for delaying his service clock, but it could at least give them some plausible deniability.