The following sentence is from NY Times. "Another corporate structure being exploited now more than ever is the master limited partnership." In the sentence, the concept of "master limited partnership" is first introduced. So, It does not have any contextual clues to identify what that refers to. But still it has "the" in it. However, the "the" in the sentence might not be used generically as the generic use of nouns is pretty much limited to nouns of animal, plant, or inventions. Then, what is the function of "the" there? What is the difference between the sentence and when I just say "a master limited partnership" or "master limited partnerships"?
The answer in John Lawler's comment is correct:
It's a Definite Generic Noun Phrase. It's an abstraction.
You suggest other possibilities:
Another corporate structure being exploited now more than ever is a master limited partnership.
This is a bit confusing and not abstract enough. It seems to imply that there is a corporate structure somewhere, that is, a particular legal entity organized as a master limited partnership, and that it is being exploited.
In other words, using the "tiger" example from John Lawler's link, we want to denote the entire species called "master limited partnership" rather than a particular member of the species. The indefinite article makes it look like we're talking about a member of the species.
Another corporate structure being exploited now more than ever is master limited partnerships.
This is less confusing and is even arguably correct, in terms of its meaning. However, the unbalanced grammatical number of the singular subject and plural predicate is somewhat awkward. If there were multiple structures being exploited, it would work: "Other corporate structures being exploited now more than ever include master limited partnerships and freeble glarb corporations."
In this sentence "the master" has the same meaning as "the main" so "the" is obligatory here