Why should we teach English phonetics and phonology?

Unlike most European languages, English is not a phonetic language: words can have identical vowel letters and be pronounced in different ways, e.g. great, treat, and threat; viceversa, words with phonologically identical vowels may have different spellings, as in sea, seize, precede. Hence the importance of teaching both pronunciation and spelling.

If we go down to the phonetic level, English appears to be idiosyncratic too, since it includes various features that are unusual from the point of view of universals:

1. a large and elaborate vowel system including complex processes of length alternation and weakening (think of compete, competitive and competition). Maddieson (1984) has estimated that only 4.1% of the world's languages have over 17 vowel sounds. RP, for instance, has about 20, which makes its system one of the less common and more complex types.
2. a consonant system that includes dental fricatives - not very common sounds in the world's languages (see John Wells's blog here) - and voiced sibilants, which tend to be problematic for many learners.
3. word stress placement that is free, i.e. arbitrary and frequently unpredictable.
4. an intonation system that seems to be more complex and to have a much higher functional load than that of most other languages.

It is because of such considerations that some have argued that for international purposes we ought to use Esperanto rather than English. For reasons that I'm not going to discuss here, this has never worked and English has become the language we use internationally.
Nowadays, everyone is expected to speak a bit of English, but trying to find someone in Italy who can speak it 'clearly' - intelligibly -, without mangling it, is no easy task at all. That is where phonetics and phonology comes in. Phonetics and phonology is a subject which is generally not taught in schools or universities in Italy, and if it is, it is taught badly. I think EFL students and teachers should both have a thorough knowledge of the phonetics and phonology of English so that they can understand native speakers better and can make themselves understood, thus avoiding misapprehensions. As Peter Roach (2009, p.6) has highlighted in his book English Phonetics and Phonology: A Practical Course (4th edition),

Pronunciation exercises can be difficult, of course, but if we eliminate everything difficult from language teaching and learning, we may end up doing very little beyond getting students to play simple communication games.

As far as native speakers of English working in ELT are concerned, they too should have a knowledge of English phonetics and phonology (most don't!). This would help them understand their students' problems much better. It's like medical diagnosis, Peter Roach has said in one of his interviews with Anne Linthe from Cambridge University Press: "if somebody came to you and asked for advice because they were getting headaches, or they were getting persistent pains in their knees or something, you could think of some homely advice that you would give them and it might well result in them improving. But it's not the same as knowing how the body works, and the nature of human illnesses and that kind of thing".
None of my past teachers ever taught me phonetics and I started to get interested in the subject only at university. It was not until then that I realized how fascinating this subject was and how essential it is for any student wanting to learn (and understand!) a foreign language - especially English!