I don’t believe that there is one correct way of speaking and that those who don’t use it are uneducated and/or stupid. Language is a fantastically complex human activity. One formal style is not flexible enough for the full range of human communication needs.
However, academic writing for conferences or journals is a special case. Using formal English is like putting on a good outfit and brushing your hair for a job interview. If you clearly care about correct style, this implies that you also care about other good research practices, such as doing a thorough literature review or ensuring your data are accurately reported. This makes the reader more disposed to believe what you are saying, which is to your advantage.
Formal English should also be used because academic writing is intended for an international audience. It isn’t respectful to ask readers with variable levels of English to puzzle over slang terms and idioms in order to understand the actual ideas presented in a paper. In contrast, formal English has been carefully formulated to be precise and clear. Ultimately, it’s to also your advantage to ensure your research is as widely disseminated as possible among everyone, and that includes the vast number of non-native speakers.
Finally, academic publishing is extremely competitive. If you want to be published, it’s best to use every trick you can to make your paper look better than the other papers submitted for publication.
In real life, formal English is not necessary or even a good idea. In an academic paper, however, I recommend keeping the language formal.