I suppose the down side is, that because it is such a rough approach, that one can overlook so many things. Especially me who doesn't seem to have a knack for details, unless I sort of engage in that kind of super focus. Applying it makes it rough with the intent for better and the efficiency of handling. But then I lose the details. I think it should be up to the person to zoom in and find it.
I still think, there are many positives compared to negatives from MBTI, because it is a tool, one of many that can help explain differences in opinion and state of mind of the individual.
Manipulation? Probably if the intent of the person was to manipulate from the beginning. It wouldn't matter, MBTI would be one of many tools to get the deed done. Manipulation is negative, while influencing is positive. Both are to the benefit of the person doing it.