How much are phase-contrast and dark-field microscopy used in biological research?
I use phase contrast very often, as it is only a matter of switching a lens group from bright field. Instead of spending hours with staining, or, worse, immunostaining, I get the big picture (cell diameter) in a few seconds. I can even go back to the same cells next day. Switching to dark-field is also a matter of turning a dial or pushing a switch.

If you already use brightfield, there is not much to learn about them in terms of actually useful physics, or sample preparation. The things you could learn - say, that colors are altered in darkfield - will rarely be needed in practice. These methods are the sort of things you don't really need to know in depth unless your experiments require them. Even then, practice and elbow grease would be more helpful than advanced optics. Remember that in most cases, the scientific hypothesis is more important than the tools you can use to solve it.