Problems
While most people would agree that some kinds of prescriptive teaching or advice are desirable, prescription easily becomes controversial. Many linguists are highly skeptical of the quality of advice given in many usage guides, particularly when the authors are not qualified in languages or linguistics. Some popular books on English usage written by journalists or novelists bring prescription generally into disrepute by making basic errors in grammatical analysis. Even when practiced by competent experts (as in text-books written by language teachers), giving wise advice is not always easy, and things can go badly wrong. A number of issues pose potential pitfalls.

One of the most serious of these is that prescription has a tendency to favour the language of one particular region or social class over others, and thus militates against linguistic diversity. Frequently a standard dialect is associated with the upper class, as for example Great Britain's Received Pronunciation. RP has now lost much of its status as the Anglophone standard, being replaced by the dual standards of General American and British NRP (non-regional pronunciation). While these have a more democratic base, they are still standards which exclude large parts of the English-speaking world: speakers of Scottish English, Hiberno-English, Australian English, or AAVE may feel the standard is slanted against them. Thus prescription has clear political consequences. In the past, prescription was used consciously as a political tool; today, prescription usually attempts to avoid this pitfall, but this can be difficult to do.

A second problem with prescription is that prescriptive rules quickly become entrenched and it is difficult to change them when the language changes. Thus there is a tendency for prescription to be overly conservative. When in the early 19th century, prescriptive use advised against the split infinitive, the main reason was that this construction was not in fact a frequent feature of the varieties of English favoured by those prescribing. Today it has become common in most varieties of English, and a prohibition is no longer sensible. However, the rule endured long after the justification for it had disappeared. In this way, prescription can appear to be antithetical to natural language evolution, although this is usually not the intention of those formulating the rules. This problem is compounded by the fact books which gain a following can remain in print long after they have become dated. This is the case, for example, with Strunk & White, which remains popular in the United States although much of its text was formulated in the 19th century.

A further problem is the difficulty of defining legitimate criteria. Although prescribing authorities almost invariably have clear ideas about why they make a particular choice, and the choices are therefore seldom entirely arbitrary, they often appear arbitrary to others who do not understand or are not in sympathy with the criteria. Judgments which seek to resolve ambiguity or increase the ability of the language to make subtle distinctions are easier to defend. Judgments based on the subjective associations of a word are more problematic.

Finally, there is the problem of inappropriate dogmatism. While competent authorites tend to make careful statements, popular pronouncements on language are apt to condemn. Thus wise prescriptive advice may identify a form as non-standard and suggest it be used with caution in some contexts; repeated in the school room this may become a ruling that the non-standard form is automatically wrong, a view which linguists reject. (Linguists may accept that a form is incorrect if it fails to communicate, but not simply because it diverges from a norm.) A classic example from 18th-century England is Robert Lowth's tentative suggestion that preposition stranding in relative clauses sounds colloquial; from this grew a grammatical dogma that a sentence should never end with a preposition.