I did send an email to MBCR Customer service before I posted on Railroad.net, and this was the reply that I received:
Dear Mr. O’Brien,
I have read your email regarding the MBTA’s choices for naming the Plymouth/Kingston and Middleborough/Lakeville lines under one heading, Old Colony, and will attempt to respond on their behalf.
The Greenbush line, while part of the original 3 Old Colony lines, was reopened just two years ago so it was determined that it should have its own timetable and would be called the Greenbush line. The reason for this separation is quite simply that the other 2 lines (of the former Old Colony) reopened some 12 years ago with great anticipation by the locals, schedules were designed to accommodate 2 lines and named the Old Colony because of its rich history and familiarity to those that would be using it. Further to that, it would be far too cumbersome to read the schedules by adding the Greenbush as the 3rd line. As for the MBTA’s website referring to these 2 lines as Middleborough/Lakeville and Plymouth/Kingston in opposition to the way the hand held schedules are designed is for clarity for someone visiting the website, like yourself, who is unfamiliar with the area. If you are in the station looking for a schedule, there are personnel at the information desk who can assist you.
I really don't think they got my point at all. Or they just don't care. It's like telling someone from out of town: "Take a left where Goldberg's used to be". "Old Colony Line" means nothing to more and more people every year.