I think it will be wise to spend some time on some of these more obscure topics... what are they, though? What topics do people often describe as "low yield"?
The high yield, low yield stuff would be nice if the MCAT tested a broad range of subjects on every exam. Unfortunately, this isn't the case. It seems that individual exams like to test you extensively on a few subjects, while most of the "high yield" stuff is left out. This can work to your favor, or it can screw you. You actually don't have to master every concept. Just be familiar/functional with everything. Very few questions require a deep knowledge of the material. They just require you to recognize simple concepts disguised in complicated passages/question stems.
Though I wholeheartedly agree with the sentiments expressed so far (that sufficient knowledge of and comfort with every testable topic is paramount for the highest possible score), I might throw out a couple topics that, as the OP said, might commonly be seen as "low-yield" for various reasons. In my opinion, topics so perceived might include:
AC current
Fungi
Organic nomenclature
Cartilage
Connective tissue
Though I wholeheartedly agree with the sentiments expressed so far (that sufficient knowledge of and comfort with every testable topic is paramount for the highest possible score), I might throw out a couple topics that, as the OP said, might commonly be seen as "low-yield" for various reasons. In my opinion, topics so perceived might include:
AC current
Fungi
Organic nomenclature
Cartilage
Connective tissue
Young's double slit experiment.
The eye, skin, and bone are probably considered low yield