Alejo, prescriptive ideas concerning language were quite common amongst linguists for a long time. Nowadays, the preference is to describe language, without making any value judgments about the language itself or the people who use it.
However, that being said, we linguists are aware that certain linguistics formulations are deemed as "correct" and "prestigious", while others are often pejoratively associated with people of low intelligence, poor expressive capability or from cultural backgrounds which we may like to stereotype.
As I've stated before in another of your questions, the use of language is dependent, to a large degree, on context. Saying 'I ain't got no money' or 'Him and me is going now' are perfectly comprehensible and fine to use in an informal situation where one is not concerned with the use of so-called "correct" grammar. But I would caution someone against using such a formulation, or such informal language in general, in other areas, such as in academic writing, applying for employment or when absolute clarity is required.
As evidenced on here, there are a large number of people who seem hell-bent on proclaiming that certain language formulations are "wrong", "bad grammar" and other such things. While these formulations may in fact go against standard conventions of grammar for the English language, these comments often fail to take into account that the use of language is governed by context, and we have different "registers" or levels of formality, and certain formulations may only be incorrect if used in an inappropriate setting.
Grammar books are prescriptive for a very good reason: the informal registers of all languages are under constant change, whilst formal registers tend to show more resistance to change. Therefore, it is easier to teach something if one knows that in a year that formulation will still most likely be in use; but we cannot always say that for informal language. And if a foreign learner of English uses a formal register of language where a native speaker might use an informal one, that's fine, as most native speakers would comprehend it without any difficulty, and would hopefully be understanding regarding someone not being a native speaker.
So, to conclude, descriptive and prescriptive views on language are both acceptable, provided that, if someone adheres to the latter, they realise the contextual capabilities of language. I think people often go too far on here, especially, judging by many of the comments I see regarding language usage. But if you can not only learn English but also learns the pragmatics of using different registers or formulations, then you'll be well on the way to being a better speaker.