Take the personal pronouns in English, for example. We have singular and plural first person pronouns: I (am) and we (are). We have three singular (feminine, masculine, and neuter) and a plural third person pronoun: he, she, it (is) and they (are). When we are talking about things, we need to distinguish between males, females, individuals and groups. Think of the vast difference in the implications of "I love each other" and "we love each other;" "she is expecting" and "he is expecting." We need these distinctions to avoid appearing ridiculous.
When we get to the second person pronouns it is odd to find only one: "you love each other" may or may not make sense since "you" may be singular or plural. If you are talking to Rhonda and her brother is present, the success of the evening could depend on whether the "you" is singular or plural if you ask, "Will you go out to dinner with me tonight?"
English curiously lacks a singular-plural distinction in the second person even though all other European languages have both: French ("tu" versus "vous"), German ("du" versus "ihr"), Russian ("ty" versus "vy"), and so on. Some time ago, English distinguished between "thou (art)" and "you (are)." "Thou" was second person singular and "you" was second person plural. Somewhere in the shuffle of history that crucial distinction was lost.
In the Southern US states, however, it would be downright unneighborly to invite your friend over without inviting the whole family-even suggestive if your are a male inviting a female. 'You (singular) come some time' would be culturally very unsouthern, so even when talking to one person, a Southerner will say "Yall come." (It is not because they don't know the difference between singular and plural.)
Even up north, "I want you to finish up" is ambiguous in an office with several people who could be responsible for finishing up. We need to distinguish singular and plural when we address people in the second person, too, but 'acceptable' English oddly does not provide for this distinction.
The speakers of English, however, have a remedy-several, in fact. Various British dialects distinguish between "you" and "you lot." Down south and in the US Midwest, the phrase "you all" is so consistently used, it has been reduced to a single pronoun: "Yall come when you can." In careful speech, Southerners will still say "you all," but in normal speech this phrase has been reduced to a single word, "yall," on a par with "I", "he," "she," "you," and "they."
Many Pennsylvanians distinguish between "you offen the light" and "yuns offen the light," where "yuns" is derived from "you ones" (analogical with "young'ns," "big'ns," "little'ns" in many dialects). Other Pennsylvanians, along with New Yorkers and folks from New Jersey, simply pluralize "you" the same way nouns are pluralized, by adding an [s]: "youse" (or, more politely, "youse guys"). Again, "youse" is a single word on a par with the other pronouns of English.
These words do not reflect ignorance on the part of speakers who use them, but a legitimate linguistic development called leveling by morphological analogy, whereby missing pieces of the grammar are generated by analogy with other parts of grammar. As languages develop, pronouns and other markers of grammatical functions are created from regular words by a process called grammaticalization. This process follows a standard route: full word > grammatical function word > affix (suffix or prefix).
An example of a grammatical function words becoming affixes may be found words like "young'n," where the pronoun "one" has been reduced to a suffix making nouns out of adjectives. The other English pronouns provide more examples. Whenever we say "gimme" rather than "give me," or "hittem" instead of "hit them," or "gitter" in place of "get her," we are using the pronouns as suffixes.
In many languages, a pronoun direct object like "me" and "them" is marked by a suffix on the verb rather than a separate pronoun. Most Native American languages do this, in fact. English will probably never follow that pattern, since it is losing it current suffixes, but it is flirting with the idea right now.
As for "yall," "youse," and "yuns," the social prejudice that made them grammatical outlaws still holds. Unless we all decide on one of these forms and all begin using it at the same time, the price of social prejudice will have to be paid by users. From a linguistic point of view, however, these forms are in fact normal and quite predictable developments that are neither right nor wrong. Guess what linguistics has to say about ain't.