Here are some advantages:
- There is a lot of existing infrastructure, resources, and training available for teaching 'Standard English'. Apart from government education departments, the big US- or UK-owned publishing houses, and agencies such as the British Council, BBC, and US State Department, all use and promote 'Standard English'.
- 'Standard English' is familiar (to most teachers and to some learners too) through broadcasting, public documents, the media, literature, etc.
- Accuracy in 'Standard English' is what most tests test; many learners learn English to take tests. It’s much easier to test knowledge of a single system of forms than to test how effectively learners can perform different communicative functions.
- 'Standard English' is expected for study and work in native-speaker cultural contexts (e.g. English for Academic Purposes in universities and colleges, or Business English in commercial settings).
- 'Standard English' is strongly associated with prestige and socio-economic power in the public mind and so is what many learners expect (their fee-paying parents too in the case of children).
- 'Standard English' is codified to a great extent (in dictionaries, grammar books, style and usage guides, etc.), thus constituting a uniform reference point for learning/teaching and testing.
Here are some disadvantages:
- 'Standard English' is actually unattainable for most learners (see Unit 3: Learning Englishes). This implies a deficit model of learning and testing, since achievement is, in part, inevitably measured negatively in terms of how short of 'Standard English' the learner falls, rather than positively, in terms of how much the learner can successfully communicate.
- 'Standard English' is inadequate and/or inappropriate for many local contexts and needs. For example, using idioms that are well-known in Australia or Ireland, like get your goat ('make you annoyed') and a piece of cake ('simple'), may not be very helpful at a Hong Kong business meeting conducted in English between Mexican native speakers of Spanish and Chinese speakers of Cantonese.
- The insistence on 'Standard English' can be very alienating for non-native speaking teachers (NNSTs), who may themselves have 'non-standard' accents and use local, 'non-standard' features of English. In the words of (Llurda, 2009), this has the effect of "reducing NNSs to perennial language learners and depriving them of recognition as legitimate language users […]" (p. 129).
- Most native-speaker teachers (NSTs) don't consistently use 'Standard English' either. Many have local accents and use local dialects inside and outside the classroom. They too can feel alienated by an insistence on a 'Standard English' which implicitly devalues their own ways of speaking (see Barrata and Halenko, 2022).
- And of course this is true for learners too. Increasingly, learners are exposed to different kinds of English outside the classroom, and often these forms are more familiar to them than those of 'Standard English', especially when this is taught through cultural contexts and practices with which they don't identify. So learners too can feel alienated by the foreignness of 'Standard English'—and especially in places where the international actions of US and UK governments (and their native English-speaking allies) do not necessarily enjoy popular local support.
- An insistence on 'Standard English'—one set of English forms among others—inevitably places a high value on forms, at the expense of the functions that these forms are used to achieve. Language has evolved not for its own sake, for the forms it takes, but because in the absence of telepathy, it's the only way we can get complex things done together in social contexts. In other words, it's evolved for the social functions it needs to serve.
A related point is that insisting on a single correct form of English prizes accuracy, but this often comes at the expense of fluency: users’ ability to express themselves effortlessly and quickly.