It's a colloquial form, and to be honest, it's not typical. You'd need to have a long, comfortable relationship with the other person plus a certain "don't care" attitude towards English. You can see in the film that this is the relationship between Jules and Vince. English speakers normally use standard forms, especially with other speakers who they don't know well. Using lazy forms with strangers just makes the speaker look uneducated and low-class. This means yes, you should avoid these lazy forms, unless you have a close group of native English-speaking friends who actually talk like that. ;)

разговорный стили и / или необразованность... грамматически это неправильно

Hello to you from the US You are correct. That is what we call "bad" english. They are using "slang" or what many would consider under educated english. This is common in gangster shows and with low income cultures. It would not be considered good english if you used it with average people but they would understand you perfectly. Your english is quite good. Keep it up. Shari