Premise: Rules should be applied consistently within a system.
Explanation: Inconsistency in the application of rules results in both a lack of clarity for the observer and an inherently self-contradictory system — in short, it isn’t a valid system at all.
Premise: A sentence is a system.
Explanation: Sentences are systems of words which, in relationship to one another according to rules of grammar, conveys the emergent property of meaning.
Premise: A paragraph is a system.
Explanation: Paragraphs are systems of sentences which, in causal and elaborative relation to one another, give rise to greater detail and more ability to elaborate upon abstract concepts.
Premise: A document is a system.
Explanation: Documents are systems of paragraphs which, in relation to one another as focus discoursive divisions of the document, mutually support the purpose of the document’s composition.
Given the rules of grammar, where referring to something in the past tense and in the present tense in two separate clauses without implying separate chronological contexts, one does not construct a valid system by composing a sentence that employs an inconsistent sequence of tenses. That is to say that “I will did that!” is an example of an invalid grammatical system, simply because the sequence of tenses (“I [future tense] [past tense] that!”) is inconsistent.
While it is easier to shift chronological context within a paragraph, and even easier within a complete document, so that a valid sequence of tenses can be observed without always using the same tense, there are still definite requirements for a valid sequence of tenses that can be violated.
Similarly, subject-verb agreement is a matter of consistently applying grammatical rules to construct a valid system. Such consistency is most often violated when someone fails to make the inflected forms of a noun and a verb match up for plurality. An example of a failure of subject-verb agreement would be a statement suck as the following:
The national budget is $3.5 trillion, which are a lot of dollars.
That should say is, not are. This is because “$3.5 trillion” is a measurement, which is a (collective) singular noun. By the same token, one says “three hundred miles is a long distance to walk,” rather than “are a long way to walk”.
Now consider the case of a company or band, or some other organization. A radical descriptivist interpretation of subject-verb agreement suggests that this is a reasonable use of the language:
Exxon Mobil Corporation aren’t very popular any longer, thanks to the 1989 oil spill.
The use of “aren’t” here implies that “Exxon Mobil Corporation” is a plural noun. Let’s try making it a more generic reference:
The corporation aren’t very popular any longer, thanks to the 1989 oil spill.
Okay, that might sound a little clumsier, and raise arguments that I’ve changed the subject from one type of noun to another. Let’s use the second type of noun, though, in a formulation that is often used in this manner by the aforementioned radical descriptivists:
The band aren’t going to the after-party for their own CD release.
Consider, however, that the same organization could be referred to using a less particular particle:
A band aren’t going to the after-party for their own CD release.
That sounds like a load of grammatical hash. It doesn’t work. It’s clumsy and obviously wrong. Even those radical descriptivists would probably agree. Let’s examine two adjacent sentences that could appear in a paragraph, though:
A band made an announcement. They aren’t going to the after-party for their own CD release.
In this example, a band (treated as singular) is referred to in the first sentence, and a pronoun for the band (treated as plural) is referred to in the second sentence. These radical descriptivists probably don’t see any problem with this, but other, similar grammatical disagreements within a single paragraph would drive many of them up the wall. Somehow, to such radical descriptivists, the rules are inviolably set in stone, except that when “everybody does it” it suddenly becomes okay.
Given the progression from sentence as grammatical system, through paragraph as system of sentences, to document as system of paragraphs, consistent application of the rules of grammar is necessary to construct a valid system. This should, in principle, apply not only to the actual manifestations of grammatical rules within the system, but the potential uses of rules. Because there are cases where a collective singular noun such as “Exxon Mobil Corporation” or “Motörhead” must be referred to using singular pronouns and verbs, plural pronouns and verbs used in the same way are less correct — because they imply an inconsistency in the application of rules within the system.
The real fun, of course, comes when the radical descriptivists’ position on something like this — the position that a valid system is not a requirement for “good” grammar — becomes the normative position. Particularly in certain regions, such as England (ironically the home of people who use phrases like “the Queen’s English” and other look-down-the-nose descriptors to imply nobody else does English correctly), the use of a collective singular noun with verbs and pronouns implying plurality is popularly regarded as perfectly acceptable.
That doesn’t make it correct, though. No number of Google hits, no equivocations by descriptivist linguists, and no amount of hand waving will change the fact that a system constructed via inconsistent application of rules is an invalid system.