Singular or plural?
They - the lexicographers, not the pronoun - decided that it was OK that THEY (the pronoun) takes a plural verb on the grounds that YOU is also both singular and plural and takes a plural verb either way. Hmmm. There IS a difference. When you use YOU, the person(s) to whom you (the person not the pronoun) speak knows whether he, she or they (the YOU to whom you speak) is one or more than one. When you use THEY, the person(s) to whom you speak is different from the person(s) about whom you speak. He, she or they (the hearers) may not know whether he, she or they (the subject/object of your utterance) are one or more than one.
2. Because "he" was rejected for unknown or unspecified genders, and "she" was too new to some ears, many writers and speakers had schooled themselves to replace singular nouns with plural ones in order to use "they" thereafter. But that often led to ambiguity if there was another plural noun in the sentence. I discussed this in Ne-ner-nis (Part 2) (scroll down to "Natural Superiority"). Using "they" as a singular, makes the ambiguity a permanent feature of the language.
The Extinction Problem
As with natural species, these days we are losing words faster than we are gaining them. That is because when we use a word that has a specific meaning in place of another perfectly fine word with a different meaning, we lose the unique meaning of the first word and have to resort to multiple words to achieve what we had before done with a single one. (See The Reticent/Reluctant Hesitation.
The Veterinarian Problem
We need a neuter pronoun not only for transgender humans but also for animals. We also need one when (1) we speak of a human whose gender is unspecified, unknown or irrelevant and (2) it is logical to speak of that human in the singular. As to animals, the other inventor of ne-ner-nis, whose invention is independent of mine and preceded it by several years, was in fact a veterinarian named Dr. Al Lippart. If it's embarrassing and offensive to call Spot "he" when Spot is a "she" or vice versa, will it be better to call Spot "they"? I invite Dr. Al to weigh in on this question.
January 31, 2016, rev (minor) 7/13/16