Quote from llg
Some schools (and some former students) cling to that "1 and only 1 theorist" approach too tightly. Others choose 1 to provide an organizational structure to their curriculum, but allow room for the use of a variety of theories. The problem when people cling too tightly to one theory is that they sometimes become trapped by it and can't think outside the parameters of that one theory. They also fail to benefit from what other theories have to offer. Finally, it goes against the very nature of science -- which is always evolving. By limiting yourself to 1 and only 1 way of thinking, your mind starts to close instead of opening up to the many possibilities out there.
As someone who has degrees in both biological sciences (Microbiology) and social sciences (Psychology), I think I can confidently say that how theory works varies depending upon the "scientific" subject involved. Understanding how to operate in one theory, I think, is hard enough; it does, as you say, give structure to the curriculum. I have not detected anything in my nursing courses, directly, that involves even understanding the "scientific method" (nor did any of the classes I had to re-take as pre-reqs to the nursing program). You need a framework to start with.
Rather than thinking it is against the very nature of science, I would say that having a theory provides the context by which something is understood, and how things are determined to be studied. If you read Thomas Kuhn's "The Structure of Scientific Revolutions" you will find that, generally, there is one predominant theory in a particular area of science at a time.
Learning the predominant theory is undergrad work; learning other theories and how to question them is grad work. Nursing science is, perhaps, unique in having a number of theories which are considered to be somewhat on a par with each other. I would maintain, however, that learning one nursing theory is the subject of undergrad education and learning substantially
about other theories is grad work.
In short, people need to learn to walk before they can run. Learning one theory is walking; learning substantially about another theory, sufficient to compare and contrast, is running.