It is if you want to feed more people, More people may not be a good thing, but it often just is an inescapable reality and if you want to keep them from death by starvation you need higher yield.
And I'm not sure rejecting monoculture on a patch of land is high yield. Not if you want to harvest and distribute it all and do it before it spoils and the "patch" is large enough. Then you need machines, huge machines, and those demand monoculture.
Or you could go with human labor but then you'd need a whole other patch of land, harvested with machines, to feed the laborers harvesting the first patch.
Corn is a good example of high calorie/poor nutrition. But most other crops are not that bad.
High yield is important if you want to feed people outside your locale. It becomes less an issue of feeding the masses and more a vehicle for trade. Through modern techniques and distribution, oranges from Florida can be sold in North Dakota. However, I wonder if it's necessary that North Dakotan's need many cheap oranges. There are many examples. What is the actual value of Columbian asparagus in the fall vs. the residual costs of producing and distributing that asparagus? Et cetera.
The labor issue you bring up is an important one, however, and it is a trade-off that many proponents of green agriculture too easily overlook.
That is to say, whith great income disparity, even when millions of people can't get enough calories, we can be throwing away moldy oranges.
Shortages of food will only make it more expensive for us, but people around the world will starve.
And a lot of people are not eating specialty foods from far off, they are barely getting enough calories from high yield starchy local crops like rice, corn, wheat etc. I am not sure how you could provide a population center like say Kinshasa with enough calories, never mind trace nutrients, without machines. Manually harvesting wheat or corn is not a way to feed the world.
I don't know if the difference is because of different species of plants or different light conditions or different soil. Whether it's better for your health I couldn't say.
Secondly, is high yield even a good thing? Many crops produce more yield but do not produce more (or produce very little more) nutritionally valuable product. Corn being the exemplary plant in this case. Germ size has barely increased, while starch has grown considerably. Which is great if you're feeding cattle, but biomass from starch doesn't necessarily mean protein-rich biomass, rather it becomes animal fat. Delicious animal fat, to be sure, but not necessarily a good yield/nutrition trade-off.