Why do so many languages use gender?

Pssst. Check post #33, at least.
And IMO, I tend to agree with For You that some of the repetitiveness and complexity of language is an error-correcting feature (like checksums or parity bits for you oh-so-logical computer types). Think about how often noise has made you miss one or two words in a sentence, but you were able to fill in the missing word by context.

In my version of English, the simple utterance “read” (present tense) would generally be interpreted as an imperative. In the past tense form, it would be taken as a passive participle (shorthand for “(the material) has been read” – by whom to be inferred from context). In written form, context would be key for interpretation.

Spoken English can be very muddy, with many of the structure words minimally audible. For instance, the first syllable in “I read …” often sounds like a glottal stop or faint vowel glide, with “you read”, one may hear little more than a “y” attack just preceding the verb. The listener picks up enough of each sound to obtain the meaning, with the most important words in the utterance given the greatest clarity. I suspect most other spoken languages work in a similar sort of way, though I know some rely on re-ordering words as emphasis.

Written language is a whole nother animal.

In English, “you” will have different tone and duration depending on context. However in Thai, tone and duration are intrinsic to a word. Some words are always pronounced with long duration and an emphatic-sounding tone (perhaps incorrectly giving a sense of emphasis to an untrained foreign listener).

That Thai generally pronounces its words more precisely than English does may help make up for a relative lack of redundancy in other aspects of Thai.

What do you mean by grammatical plural–do you mean that the /s/ ending is required even though the plurality might easily be inferred from the context?

I think a better example of a seemingly arbitrary rule would be the way articles are used. Even just among IE languages, many of them have their own rules concerning when you can or must use an article, and when not. In English this often turns on whether the noun is plural or singular. Consider the following sentence.

[ul][li]A cat is an instinctive hunter[/ul][/li]
but the closest parallel with two or more cats would be

[ul][li]Cats are instinctive hunters[/ul].[/li]
..that is, you have to drop the article. Although the following sentences are perfectly correct grammatically…

[ul][li]Some cats are instinctive hunters[/li][li]Many cats are instinctive hunters[/li][/ul]

…you would be changing the meaning significantly by using them, because many and some imply the allusion to only a subset of all cats.

In English we don’t use articles with proper nouns like “John” or “Ralph” or “Professor van Leeuwenhoek”, but in German you can. For instance,

[ul][li]Wie geht’s dem Hans?[/ul][/li]
means “How’s it going with Hans?” (Literally, “how goes it to the Hans”).

Because sex feels good!

And Men want it!

Real bad!

Women, too!

And we have gender so we don’t get, say, a peanut butter sandwich, by accident.

Oversimplified, but with much truth at the core.

That’s right, but that’s English. In another language the simple utterance “read” could mean the equivalent of “I read,” while another utterance could mean “You read.” It’s impossible to express this in English precisely because we can’t do this in English.

Some languages, like Japanese, can leave out pronouns if they are understood from the context. For example, if you’re talking with your boy/girl-friend, and said “Daisuki”, meaning “Love”, it would be understood as “I love you”, even though you left out both subject and object of the verb.

You can also say:
[ul][li]Few cats are instinctive hunters.[/ul][/li]
which means an even smaller subset of cats. But then you can add the indefinite article:

[ul][li]A few cats are instinctive hunters.[/ul][/li]
Which has an entirely different connotation. The former, without the article, has an unstated assumption that all cats are instinctive hunters; the statement is contradicting that assumption. The latter, with the indefinite article, comes from the opposite assumption that there are no cats that are instinctive hunters, but there are indeed a few that are.

And then you can replace the indefinite article with the definite article:
[ul][li]The few cats [that] are instinctive hunters…[/ul][/li]but that is an incomplete sentence so I won’t elaborate further.

[Aside]
Heh. Around here where I live, English is often referred to among the school set and people who fancy themselves wits, when fancying themselves witty, as *“el difícil” *-- “the difficult one” (As in, “You have to tell him both in Spanish and in The Difficult One”).

Of course the only thing distinctly “difícil” about it in this case is that it’s a language that everyone has to sit for a required class in school when they’d rather be doing something else. The only thing “easy” about Spanish is that we learned it on our mothers’ lap by absolute immersion when our little brain was at optimum readiness for acquiring language skills.

Otherwise Spanish has, just to mention a few: grammatical gender, including inanimate objects, for both adjectives and articles; TWO forms of “to be”; reflexive verbs; seventeen distinct-form verb tenses (not counting the infinitives and gerunds) and in turn up to six forms for combinations of person and number in each of the tenses, depending on the verb.

Yet people claim it’s English that’s hard, just because when written a number of the letters do not always represent the same phoneme…
[/Aside]

Plenty of languages get along just fine without having to change words to mark whether a noun is plural or not. Chinese, for example. Chinese has one plural word (men) which is used only with people. To indicate how many there is of an object, Chinese hit on the simple solution of numbering them. So…

wǒ de shū - 我的书
My books

wǒ de shū - 我的书
My book

On the subject of gender, Chinese has done something unusual. As in so many other gender-free languages, the word for both ‘he’ and ‘she’ is one and the same: . However, they’re written with different characters:
he: 他 (left radical is the character for ‘person/man’ 人)
she: 她 (left radical is the character for ‘woman’ 女)

Korean used to be gender-free. But in the 20th century, they adopted different-gendered third-person pronouns, apparently under pressure from European languages. Originally, ‘he’ and ‘she’ were both just
.
But now, while they’ve kept the original pronoun for ‘he’, they’ve made a new one for ‘she’:
그녀 kǔ-nyǒ.*
So men get the unmarked form, the default. Women are marked, as a subsidiary category. Quite unfortunate how Koreans decided to import pronominal sexism from Europe when all along they’d been free of it.

*Could also be romanized “geu” and “geunyeo.”