Answer:
Theory X and Y are limited in that they are absolutes. As McGregor defines them, they are "the two ways" of viewing all workers. Perhaps in the 1950's when McGregor conducted his research, Theory X was the prevailing method of management and his idealistic (or humanistic) approach to management seemed reasonable to apply in contrast.
However, seldom do we see that one person has exclusively Theory X or Theory Y belief systems. In many cases, managers find certain people (and certain roles) are more apt to be motivated in certain ways, and through trial and error, perhaps deduce that some "would be 'Theory X'" employees actually have the drive, motivation, and intelligence to be treated differently, and even thrive under those conditions.
Source: http://www.project-management-course.info/theory-x-manager-eeek/