This discussion is vastly oversimplifying the complexity of nectar secretion. Just in our area an example would be goldenrod and clover. The goldenrod seems to yield profusely on acid soil but not on sweet soil, whereas just the opposite seems true of clover. 

Nectar secretion is very connected to weather conditions, but different plants respond differently to different types of weather. Some secrete better if it's hot and humid, others if there are cold nights, etc. Nectar secretion has evolved as one of several aspects of plants that attract pollinators. These would include floral display, odor, pollen abundance, etc. 

There is also a wide range of features which are present as deterrents, particularly against plant predators. Comparing the different honey and pollen plants serves mainly to demonstrate how different they all are. Evolution has produced an almost infinite variety of solutions to basic problems, such as plant pollination.

PLB

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