Having the different voices makes it easier for the reader to distinguish the different characters and also it brings out the character, making them more alive. I, myself have a mix voice. It is a mix of Ontario, Nova Scotia and a few words of Newfoundland from a friend of mine. But if I want a different voice, I have a hard time finding it. But writing it may not be a problem if you know where to look for the right dialect.
Speech patterns are also a way of distinguishing the different voices.
When on a bus or in a mall you can listen to the words that are being said by the people around you. Once on the bus I was listening to the words of some teenagers. One was swearing with every second word. The other looked over at me, seeing my two children, told the other one to watch her language. With the language cleaned up you could still tell it was a teen that was talking.