Answer: Undoubtedly that's true. But you've also heard people in English use the word "ain't" and use grammatically questionable sentences such as "everybody should do their best" and "it don't matter." Yet you're unlikely to see textbooks use them as examples.
In teaching grammar, most textbooks and this site place their emphasis on what is considered standard speech used by educated speakers. In practice, they tend to be even more conservative than that, using as examples what educated speakers use in formal writing. As a general rule in both Spanish and English, perhaps all languages, formal writing is more grammatically precise than spontaneous speech.
To use the example you gave, almost all textbooks will tell you that the singular form of haber is used for both singular and plural contexts when haber is used existentially, that is, with meanings such as "there are," "there will be" or "there were.' Thus the same word, hay, is used to mean "there is" and "there are" in sentences such as hay una silla (there is one chair) and hay tres sillas (there are three chairs).
The rule is invariably followed in the present indicative; thus hay means either "there is" or "there are." However, in practice the rule isn't always followed in the other tenses, especially in speech and casual writing, although usage varies by region. Thus it isn't unusual to hear or read sentence constructions such as habían tres aviones (there were three airplanes) or habrán dos elecciones (there will be two elections), to use examples from recent Latin American news articles.
As a foreigner, you're best off knowing and using the "standard" grammar until you're in an area long enough to pick up the local language peculiarities. Although habían is far from a major grammatic offense, to use a nonstandard usage in an area where it isn't the norm could make you sound miseducated or excessively informal.
