Many people argue that English language examinations are a barrier preventing students from learning 'real' English, which is the English spoken by native speakers among themselves.
I feel that this is an overstated argument and does not hold a lot of water. The job of exams is to provide evidence that a student has reached a certain level of understanding and communication. The language tested in exams is comprehensible to native speakers, even though it lacks the variety that they use. Therefore, they do guarantee that speakers will be understood.
A lot of colloquial language is shortlived and also very culture-bound, which means that it would take a lot of time to teach and would only be of use when speaking to native speakers. Given that many students will spend a lot of their time using English with other non-native speakers, it makes little sense to focus or concentrate on this.
For students who are living in an English-speaking environment, it does make more sense to look at the language being used in the streets around them, but this does not mean that it automatically makes sense for all, nor that this type of English should be tested.