We used to use greenboard completely around the tub/shower as a tile base. 30 years later, many of these are still in good condition. As long as the grout and caulk is kept intact and sealed, there is no problem. In your case, since you have backerboard 1/2 way up, it really shouldn't be an issue. Just make sure that a tile grout line doesn't line up with the backerboard/greenboard line, or you may have grout cracking problems.
If the wall is an outside wall, then I would recommend a plastic sheeting vapor barrier. If it is an inside wall, don't worry about it.