Amateur microscopy is the recreational use of microscopes. Plain and simple. A veterinary doctor who uses microscopes in order to identify parasites in the fur of animals is not an amateur microscopist. He/she uses the microscope not for recreational purposes but in order to treat an animal. Researchers who use microscopes to study the events taking place in cells are also not amateur microscopists, because they use microscopes as a means to solve a particular research problem. This naturally does not mean that they do not “like” to use the microscope. Rather the device is a tool, a means, to achieve another goal. As the name “amateur” implies, amateur microscopists use these devices for the love of it. Amateur microscopists love microscopes either for their technology, their design, their antiquity (in case of collectors) or for the love of natural observation.
The type of equipment used is not relevant for distinguishing between amateurs and professionals. Doctors (professional microscope users) sometimes use comparatively inexpensive devices, because this is all that they ever need for their routine observations. I remember one doctor who had a microscope with only one objective attached. This was all he needed. He did not even focus the image, he simply dropped the slide onto the stage and quickly checked for the presence of certain microorganisms.
Advanced amateurs sometimes invest quite a bit into advanced equipment, such as DIC (Differential Interference Contrast) which produces quite spectacular images that appear to be 3D. Some research laboratories, yet, own light microscopes which cost as much as an apartment. It depends all on the research objectives.
For some people the microscope is a meaningful addition to an already existing hobby. People who collect stamps, minerals or fossils might find stereo microscopes not only useful but also a great enhancement of their hobby.