Why Study Protists?
Besides the fact that they're diverse, what is a good reason to study protists? Well, there are lots of great reasons. Evolution, which I'll get back to in a second, ecology, when you start to look at the ocean, the basis of food chain in the ocean, the phytoplankton, which are algae, produce something like, say, 70 percent of the oxygen on the earth. And when you get toward the polar regions, the only organisms there are protists, plus some of their bacterial friends. And also for ecology, protists have been used in model systems to look at competition inlab experiments. It's a lot easier to put a couple of protist species in a jar and study their interaction, rather than following lines and links and things like that. So they're great little model systems.
For cell structure...
In terms of cell biology, protists are used quite a bit to look at cell structure under the electron microscope and so on, but even way before that protists were used to study cell structure. Really protistology grew up with the whole field of cell biology. Also, protists are used to look at the origin of organelles. I'm sure everyone knows that the current thinking is that mitochondria and chloroplasts are endosymbiosis--algae in the latter case and bacteria in the former case--and some protists have chloroplasts that are really secondary endosymbiosis. They have been injested by another protist and so they have the chloroplasts of the alga they ate, it's sort of like a Russian doll situation of an alga within an alga.