Dear friends,
I think grammar is important, of course, but what is also extremely important is to express yourself in a way that "sounds" english. For example, if you are translating your thoughts and ideas from your native language, even though you may have a perfect grammar, you would not be expressing yourself in everyday spoken english.
Also, I don´t believe that is the "must". The important issue is to create a good combination between grammar, pronunciation and english structures and collocations that make your speech understandable to the person or group of people that you are talking, and that you can also understand them.
Culturally speaking, this issue is very important!