Yes, you must use the cases. If you don't, you will not really be speaking Russian and will be poorly understood. Russians who have not been exposed to English may not understand you at all.
The case endings are not decorations. Each one of them conveys a clear picture to native speakers. Take them out of a sentence, and it turns into mush. It would be like speaking English while deliberately leaving out the words "of", "to", "from", and "for".
Using cases is nothing like using proper spelling or knowing when to say "whom". Using cases is not a sign of education or good breeding. Cases are used extensively even by small children and the criminal classes. If you want to speak Russian, you must learn what they mean and use them too.
See our article Grammar 2—Introduction to Inflection and Grammatical Case.